Geochemistry of MORB and OIB in the Yuejinshan Complex, NE China: Implications for petrogenesis and tectonic setting

Accepted Manuscript Geochemistry of MORB and OIB in the Yuejinshan Complex, NE China: Implications for petrogenesis and tectonic setting Jun-Hui Bi, Wen-Chun Ge, Hao Yang, Zhi-Hui Wang, De-Xin Tian, Xi-Wen Liu, Wen-Liang Xu, De-He Xing PII: DOI: Reference:

S1367-9120(17)30335-8 http://dx.doi.org/10.1016/j.jseaes.2017.06.025 JAES 3131

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

10 April 2017 27 June 2017 27 June 2017

Please cite this article as: Bi, J-H., Ge, W-C., Yang, H., Wang, Z-H., Tian, D-X., Liu, X-W., Xu, W-L., Xing, DH., Geochemistry of MORB and OIB in the Yuejinshan Complex, NE China: Implications for petrogenesis and tectonic setting, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.06.025

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Geochemistry of MORB and OIB in the Yuejinshan Complex, NE China: Implications for petrogenesis and tectonic setting

Jun-Hui Bi a, Wen-Chun Ge a*, Hao Yang a, Zhi-Hui Wang a, De-Xin Tian b, Xi-Wen Liu c, Wen-Liang Xu a, De-He Xing d

a

College of Earth Sciences, Jilin University, Changchun 130061, China

b

Liaoning Institute of Mineral Exploration, Shenyang 110032, China

c

Jilin Institute of Geological Sciences, Changchun 130012, China

d

Shenyang Institute of Geology and Mineral Resources, Shenyang 110034, China

*Corresponding author:

College of Earth Sciences Jilin University No. 2199 Jianshe Street Changchun, 130061, China

Tel:

86-0431-88502278

Fax:

86-0431-88584422

E-mail:

[email protected]

Abstract The Yuejinshan Complex, a remnant of the late Paleozoic western Paleo-Pacific Ocean, is located between the Jiamusi Massif and the Nadanhada Terrane in NE China. The complex consists of strongly deformed metaclastic rocks, marbles and metabasaltic lavas. The basalts have undergone hydrothermal alteration, greenschist facies regional metamorphism, and surface oxidation. These rocks can be divided into two broad rock groups based on whole-rock geochemical and Sr–Nd isotopic characteristics: (1) mid-ocean ridge basalt (MORB-type) tholeiites that range in composition from light rare earth element (LREE)-depleted varieties (N-MORB; (La/Sm)N < 1), showing highly positive Nd(t) ratios (+10.2 to +10.5), to LREE-enriched tholeiites (E-MORB; (La/Sm)N > 1); and (2) ocean island basalt (OIB-type) alkaline lavas, characterized by less positive Nd(t) ratios (+6.20 to +8.61), with significant enrichments in LILE, HFSE, LREE, and MREE, and slight depletions in HREE, relative to average N-MORB. Trace element and isotope systematics indicate that the tholeiitic basaltic rocks were derived from partial melting of a depleted MORB source in the spinel facies mantle, whereas the source of the E-MORB was a depleted MORB mantle (DMM) source significantly enriched by OIB-type components. In contrast, the alkaline basalts were generated from an enriched OIB-type mantle source in the garnet facies and continued melting to spinel facies mantle depths. Therefore, the mafic volcanic rocks of the Yuejinshan Complex, located above the oceanic plate of the Paleo-Pacific Ocean, were most likely derived from chemically heterogeneous mantle sources during back-arc basin spreading and plume-related volcanism. The westwards subduction of the Paleo-Pacific Ocean lithosphere during the late Carboniferous to middle Permian resulted in the back-arc lavas, seamounts, and other oceanic fragments accreting onto the eastern Jiamusi Massif, forming the Yuejinshan Complex.

Keywords: Yuejinshan Complex; Geochemistry; MORB and OIB; Jiamusi Massif; Paleo-Pacific

Ocean

1. Introduction

The Central Asian Orogenic Belt (CAOB) extends from the Urals in the west to the circum-Pacific orogenic belt in the east, separating the Siberian Craton in the north from the Tarim–North China Craton in the south (Jahn et al., 2000, 2004; Kröner et al., 2014; Sengör et al., 1993; Windley et al., 2007; Xiao et al., 2009, 2010a; Fig. 1). The CAOB is one of the world’s largest accretionary orogens, formed by the accretion of juvenile material during the subduction and closure of the Paleo-Asian Ocean, from the early Neoproterozoic to late Paleozoic (Allen et al., 1995; Cai et al., 2011, 2012; Eizenhöfer et al., 2014; Geng et al., 2011; Han et al., 2015; Sengör et al., 1993; Xiao et al., 2010b; Xiao and Santosh, 2014). The development of the CAOB involved the accretion of oceanic islands, seamounts, oceanic plateaus, island arcs, accretionary complexes, ophiolites, and microcontinents onto the southern margin of the Siberian Craton (Badarch et al., 2002; Buslov et al., 2001, 2004; Coleman, 1989; Eizenhöfer et al., 2015a, b; Feng et al., 1989; Han et al., 2016a, b; Khain et al., 2002; Windley et al., 2007; Xiao et al., 2010a; Zhang et al., 2016). Various oceanic fragments were preserved within accretionary complexes during the progressive closure of the Paleo-Asian Ocean by subduction. Oceanic fragments have formed in various tectonic settings, including supra-subduction zones (SSZs), continental margins, mid-ocean ridges, and oceanic islands (Dilek and Furnes, 2011, 2014). A detailed examination of the rock types within individual accretionary complexes can provide critical information on the overall geodynamic evolution of ancient oceanic basins, prior to subduction (Colakoglu et al., 2012). The Yuejinshan Complex (YC) crops out between the Jiamusi Massif and the Nadanhada Terrane (Fig. 2b), which represents remnants of oceanic crust. Previous studies have reported numerous paleo-oceanic fragments with mid-oceanic ridge basalt (MORB) and/or ocean island basalt (OIB) affinities in the Yuejinshan area, which can provide geological information on the tectonic evolutions of this region

(Guo, 2016; Shao and Tang, 1995; Tian et al., 2006; Yang et al., 1998; Zhang et al., 1997; Zhou et al., 2014). However, the emplacement time and mechanism of the YC cannot be constrained based on geological evidence from the previous study, and various models have been proposed. Some workers proposed that the YC is marked by the tectonic mélange and generated by the rifting and back-arc spending in the eastern margin of the Jiamusi Massif (Guo, 2016; Zhang et al., 2008), which was caused by the roll-back of subducting oceanic slab during the middle–late Triassic (Guo, 2016). In contrast, other workers consider that the YC, which is part of the Nadanhada Terrane (Tian, 2006; Zhang et al., 1997), represents the first stage of an accretion complex created by subduction–accretion of the Paleo-Pacific plate at some time between 210 and 180 Ma (Zhou et al., 2014). Further evidence for the 210-180 Ma assembly is derived from the metamorphism of the Heilongjiang Complex in the western part of the Jiamusi Massif (Zhou et al., 2009, 2014). Yang et al. (1998) also reported a whole-rock Rb–Sr isochron age of 188 ± 4 Ma for mica schists of the YC in the Dongfanghong area. This is mainly due to a lack of knowledge of the petrogenesis, mantle source, and geodynamic setting of the oceanic mafic-rock fragments. In this paper, we report an integrated study of major elements, trace elements, and Sr–Nd isotopes of the mafic volcanic rocks within the YC belt. We characterize the petrogenesis and geodynamic evolution of these mafic volcanic rocks of the Yuejinshan Complex using newly collected samples. The results, in combination with published data, provide new constraints on the formation and development of the Paleo-Pacific Ocean in the eastern margin of the Jiamusi Massif, NE China.

2. Regional geology

Northeastern China and adjacent regions have traditionally been considered as the eastern part of the CAOB, located between the Siberian and North China cratons (Jahn et al., 2000, 2004; Li, 2006; Sengör et al., 1993; Sengör and Natal’in, 1996; Wilde, 2015; Zhou et al., 2017; Fig. 1). The tectonic evolution of the region was

controlled by the subduction and accretion of the Paleo-Asian Ocean crust during the Paleozoic (Li, 2006; Windley et al., 2007). In the Mesozoic, the region underwent post-orogenic extension during the Paleo-Pacific and Mongol–Okhotsk tectonic systems (Wu et al., 2007, 2011; Xu et al., 2013). The result of the multiple tectonic systems is a collection of micro-continental massifs and terranes, separated from one another by major faults (Jahn et al., 2000; Li, 2006; Safonova et al., 2009; Sengör et al., 1993; Wu et al., 2011; Xiao et al., 2009; Zhu et al., 2015, 2017a, b), including the Erguna Massif in the northwest, the Xing’an and Songliao terranes in the central area, the Bureya–Jiamusi–Khanka Massif in the northeast, and the Sikhote–Alin Terrane in the east (Fig. 2a). The Jiamusi Massif mainly comprises Precambrian basement (Mashan Complex), and Paleozoic granitoids and related volcano-sedimentary rocks (Fig. 2b). The Mashan Complex is composed of interleaved ortho- and paragneisses, marbles, and graphitic schists that were deformed and metamorphosed under conditions of the upper granulite or amphibolite facies (Jiang, 1992; Lennon et al., 1997). Precise zircon U–Pb ages indicate that the granulite facies metamorphism in the Mishan region of the Jiamusi Massif occurred at ca. 563 Ma, followed by retrograde metamorphism at 518–496 Ma (Yang et al., 2017), and the unit is considered to be a Late

Pan-African terrane,

possibly derived

from East

Gondwana

during

Neoproterozoic to early Paleozoic time (Wilde et al., 2000, 2010; Yang et al., 2017). Geochronological data indicate that the Jiamusi Massif underwent multiple phases of magmatism, with the majority of the igneous rocks emplaced in the Paleozoic and minor magmatism in the Neoproterozoic and late Mesozoic (Bi et al., 2014a, 2016; Wu et al., 2001, 2011; Yang et al., 2015, 2017). The Paleozoic intrusive rocks can be divided into two phases, with earlier phase at 541–484 Ma, associated with the Late Pan-African orogenic event (Bi et al., 2014a; Wilde et al., 2000; Yang et al., 2014). The younger phase of I- and A-type magmatism, at 302–260 Ma, probably occurred in a geodynamic regime that changed from compression to extension during the westward subduction of the Paleo-Pacific oceanic lithosphere (Bi et al., 2014b, 2015, 2016; Yu et al., 2013; Yang et al., 2015). In addition, some bimodal volcanic rocks in

the eastern Jiamusi Massif were interpreted to have erupted in an arc-related extensional setting at 291–263 Ma (Bi et al., 2017; Meng et al., 2008). The Paleozoic granitoids, together with widespread volcanic rocks, constitute the majority of the massif. The Nadanhada Terrane, which is the Chinese part of the Mino–Sikhote–Alin, is located to the east of the Jiamusi Massif (Fig. 2a) and comprises Triassic–Jurassic accretionary complexes, intruded by Cretaceous granites (Cheng et al., 2006; Kojima, 1989; Li et al., 1979; Wang et al., 1986), and a Cenozoic sedimentary succession (HBGMR, 1993). The Triassic–Jurassic accretionary complex (also known as the Raohe Complex) formed as part of the Paleo-Pacific accretion belt. The complex consists mainly of limestone, chert, siliceous shale, mafic–ultramafic rocks, and clastic rocks. Recent U-Pb dating of detrital zircons from a sandstone of the Yongfuqiao Formation has youngest age populations (approximately 140 Ma) representing the maximum age of deposition (Sun et al., 2015a). Volcanic rocks collected across this region show eruption ages of 187–174 Ma and their geochemical features suggest a subduction-related setting (Wang et al., 2017). U–Pb dating indicates that the plagiogranite, gabbro, and pillow basalts in the Raohe Complex formed at 169–166 Ma; these rocks show an OIB-type geochemical signature and are interpreted as fragments of seamounts (Cheng et al., 2006; Wang et al., 2015; Zhou et al., 2014). Granites within the Hamahe and Taipingcun plutons emplaced at 131–115 Ma and are S-type granites, as indicated by the occurrence of cordierite (Cheng et al., 2006). Paleontology and paleomagnetic data indicate that the Nadanhada Terrane was accreted onto the Asian continental margin in the Early Jurassic–Early Cretaceous from low latitudes (Cheng et al., 2006; Mizutani and Kojima, 1992; Wang et al., 2015). The Yuejinshan Complex was previously referred to as the Yuejinshan ‘Group’ in the Chinese literature, defined as the remnants of a paleo-ocean and trending NE–SW between the Jiamusi Massif and Nadanhada Terrane. The YC is characterized by discontinuously exposed, strongly deformed metaclastic rocks, marbles and greenschist-facies mafic volcanic rocks that were intruded by Cretaceous granitoids

(Fig. 3). The metaclastic rocks include quartzite, quartz schist, and mica schist, and they are interpreted as continental slope sediments that experienced lower greenschist facies metamorphism (Shao and Tang, 1995). Guo (2016) established that the metaclastic rock unit consists of Mesoproterozoic–middle Triassic sediments and that the timing of deposition was later than the middle Triassic. The Rb–Sr isochron age of muscovite in mica schist is 188 ± 4 Ma, which possibly records the age of metamorphism (Yang et al., 1998). LA–ICPMS zircon U–Pb isotopic ages constrain that the basalts in the Hamatong area of YC were erupted at 232 ± 5 Ma (Guo, 2016). Zhou et al. (2014) further proposed that the YC part of the Nadanhada Terrane was accreted between 210 and 180 Ma, although no precise age data were given. The composition of the mafic volcanic rocks is predominantly similar to that of MORB and OIB (Guo, 2016; Shao and Tang, 1995; Zhang et al., 1997; Zhou et al., 2014). In addition, mafic–ultramafic intrusive rocks previously identified as the Yuejinshan ophiolitic mélange (Sun et al., 2015b) are also exposed in the YC. According to Shao and Tang (1995), gabbros intruded the YC in the Hamatong area after metamorphism and deformation of the YC. Bi et al. (2015) proposed that the Dongfanghong gabbros were emplaced in an active continental margin setting, and these rocks yield ages ranging from 290 to 274 Ma. Locally, our field geological survey found mica schist xenoliths within serpentinized peridotite, supporting the view that the peridotite is not part of the YC.

3. Sample locations and petrology

We collected 39 samples from the YC belt (Fig. 3). Mafic-volcanic samples were collected from the Qindeli and Jiangbian areas in the north (Fig. 3a), and the Nanlingongduan, Binqiaozhen, Jianshan, and Dongshan areas in the south (Fig. 3b). The sample locations are listed in Table 1. The mafic volcanic rock samples are mostly fragmented in a quarry outcrop (Fig. 4b, c), and forming a tectonic mélange with quartzite in the Nanlingongduan area (Fig. 4b). All mafic volcanic rock samples have experienced oceanic-type metamorphism

and generally show a mineral assemblage that is typical of lower greenschist facies conditions. In most cases, these metabasalts crop out as greenschists, with crystalloblastic textures, which are composed mainly of chlorite, epidote, actinolite, carbonate, albite, saussurite and opaque minerals, and the remnants of altered plagioclase and pyroxene (Fig. 4d–f). The phenocrysts are mainly euhedral and semi-euhedral pseudomorphs after pyroxene (Fig. 4d). Albite and saussurite commonly replace plagioclase and chlorite, and epidote and actinolite replace pyroxene. A strong folding deformation can be observed in sample 12GW070 (Fig. 4e). In the Dongshan area, metabasalts display primary textures, and are distinguished by some easily visible plagioclase grains preserved within a matrix composed of chlorite and carbonate minerals (Fig. 4f).

4. Analytical methods

Fresh portions of volcanic rocks were used for analysis. The rock samples were first broken into small chips using a hammer wrapped in paper to avoid potential contamination. The chips were then soaked in ~10% hydrochloric acid for at least 24 hours and rinsed with high-purity deionized water, and then dried. They were then ground in an agate mortar to ~200 mesh for major element, trace element, and Sr–Nd isotopic analyses. Whole-rock geochemical analyses were performed at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology, Beijing, China. Major elements were analyzed by X-ray fluorescence (XRF; AB-104L, PW2404), using fused glass disks according to the Chinese national standard (GB/T14506.14-2010). Trace elements were determined using an ELEMENT XR inductively coupled plasma–mass spectrometer

(ICP–MS)

according

to

the

Chinese

national

standard

(GB/T14506.30-2010). Analytical results for a Chinese standard (GDW07104) indicate an analytical precision of >95% for major elements and >90% for trace elements. Full analytical details are given by Li (1997). Analytical results are listed in Table 1.

Rb–Sr and Sm–Nd isotopic analyses were performed on a Neptune (MC)–ICP–MS at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China, following the method of Liang et al. (2003). Rb–Sr isotopes were separated using cation columns, whereas Sm–Nd isotopic fractions were separated by HDEHP-coated Kef columns. Measured 143

Nd/144Nd ratios to

143

87

Sr/86Sr ratios were normalized to

87

Sr/86Sr = 0.1194 and

Nd/144Nd = 0.7219. During the collection of isotopic data,

repeated analysis of the strontium NBS SRM 987 isotopic standard gave an average 87

Sr/86Sr value of 0.701243 ± 0.000014 (2σ), and the Nd isotopic standard Shin-Etsu

JNdi-1 gave an average

143

Nd/144Nd value of 0.512124 ± 0.000011 (2σ). Detailed

analytical results are listed in Table 2.

5. Geochemistry

5.1. Effect of metamorphism on element mobility

Most of the rock samples from the study area are characterized by moderate to high loss on ignition (LOI) and the presence of secondary minerals (Section 3). The rocks have undergone hydrothermal alteration plus accretion-related deformation and greenschist facies metamorphism, during which some elements, such as Na, Rb, Ba, U, K, and Sr (Table 1), may have been mobile (Photiades et al., 2003). Therefore, our modeling and interpretations are based mainly on high field strength (HFS) elements (Th, P, Nb, Ta, Zr, Hf, Ti, Y, Sc, and REE), as well as some transition metals (e.g., Ni, Co, Cr, and V), which are considered immobile during alteration and can be effectively used to describe the primary chemical features of a rock (Daux et al., 1994; Hart, 1970).

5.2. Major and trace element compositions

The basaltic nature of the analyzed samples can be observed in their major

element chemistry, and they can be divided into two major groups, Type-I and Type-II, based on their geochemistry, specifically the concentrations of incompatible elements (e.g., Nb, Zr, Ti, and LREE) (Table 1). To enable comparisons among samples and for displaying the data graphically, the datas were recalculated to 100% anhydrous. Type-I comprises metabasalts from the Qindeli, Jiangbian, Binqiaozhen, and Nanlingongduan areas (Fig. 3). All the samples have high Nb/Y values and plot in the alkaline-basalt field in the SiO2 vs. Nb/Y diagram (Fig. 5a). The samples also show high TiO2, P2O5, Nb, Ta, Zr, Hf, Th, and U contents (Table 1). REE patterns for Type-I samples display enrichment in LREE compared with HREE, exemplified by (La/Sm)N ratios of 1.60–3.23, (La/Yb)N ratios of 4.75–9.98, and insignificant Eu anomalies (Fig. 6a). The chemical signature is similar to that of OIB or IAB (Sun and McDonough, 1989). The Type-1 samples show high concentrations of HFSE with respect to N-MORB and display a regular decrease from Rb to Lu in normalized trace element patterns (Fig. 6b), indicative of basalts derived from a geochemically enriched mantle component in an intraplate setting. This interpretation is corroborated by high Ti/V ratios (Fig. 7a), Th/Yb and Nb/Yb ratios (Fig. 7b), and by the distribution of data on tectonic discrimination diagrams (Fig. 7c and d). Note that there is a very strong negative Sr anomaly in the spidergram diagram for these samples. It is not consistent with the characteristics of the OIB-type rocks. This may be caused by the treatment of carbonates during the clean of the rocks using the 10% HCl, because of the Sr may have migrated into carbonate during alteration. Type-II is represented by metabasaltic rocks in the Jianshan, Dongshan, Nanlinggongduan, and Binqiaozhen areas (Fig. 3b). These rocks are generally sub-alkaline (Fig. 5a), and samples from all localities mostly plot in the high-Fe tholeiitic basalt field on the FeO*+TiO2–Al2O3–MgO ternary diagram (Jensen, 1976; Fig. 5b). Type-II rocks are characterized by moderate to relatively low TiO2, P2O5, Nb, Ta, Zr, Hf, Th, and U contents (Table 1; Fig. 6d). Ni and Cr contents are also relatively low, ranging from 42 to 161 ppm and from 124 to 343 ppm, respectively. The samples display a nearly flat REE pattern, and show moderate depletion to weak enrichment of LREE and MREE, with (La/Sm)N ratios of 0.37–1.36, and insignificant

Eu anomalies (Fig. 6c). The rocks have a low amount of HFSE and show flat MORB-normalized trace element patterns (Fig. 6d). The chemical signature of the rocks, particularly the Ti/V ratios, is typical of MORB or BABB (Fig. 7a). The rocks can be further divided into variably depleted MORB (Type-IIa) and relatively enriched MORB (Type-IIb). Type-IIa rocks have lower Nb/Yb, Ce/Y, and Th/Yb ratios with respect to Zr content compared with Type-IIb rocks. In the Th/Yb vs. Nb/Yb diagram (Fig. 7b), Type-IIa rocks show elemental ratios close to the N-MORB composition of Sun and McDonough (1989), whereas Type-IIb rocks have elemental ratios ranging from those typical of N-MORB to those of E-MORB. In Fig. 7d, Type-IIa rocks collectively plot in the field of N-MORB compositions, whereas Type-IIb rocks plot in the E-MORB field.

5.3. Sr and Nd isotope compositions

The initial isotopic ratios of the samples were calculated back to 320 Ma. The Type-I samples give a range of low (87Sr/86Sr)i ratios (0.70067–0.70536) and 143

Nd/144Nd ratios (0.51282–0.51293), (Nd(t) = +6.20 to +8.61). In contrast, the

Type-II samples have relatively high (87Sr/86Sr)i ratios (0.70631–0.70711) and 143

Nd/144Nd ratios (0.51318–0.51325), (Nd(t) = +10.2 to +10.5) (Fig. 8). Overall, the

Type-I samples have lower Nd(t) and 87Sr/86Sr ratios than the Type-II samples, which suggests a more enriched mantle source. However, Sr isotope data for the basaltic rocks in this study are unusual, with ISr values up to 0.70711 and plotting to the right of the mantle array in the Nd(t) vs. ISr diagram (Fig. 8). The horizontal displacement evident in the Nd(t) vs. ISr diagram is consistent with the effect of the hydrothermal interaction of seawater with oceanic crust (O’Nions et al., 1977; DePaolo and Wasserburg, 1977).

6. Discussion

6.1. Nature of the mantle source

The geochemical analyses indicate that two chemically distinct groups of magmatic rocks occur in the YC: (1) OIB-type alkaline basalts, and (2) MORB-type tholeiitic basaltic rocks. It can be seen that variable OIBs (Type-I), N-MORBs (Type-IIa) and E-MORBs (Type-IIb) samples show the elemental ratios in Fig. 7b, plot within the trend defined by the mantle array, suggesting that their mantle source had no subduction influence and the resulting magmas were not significant ly contaminated by lithospheric material. The low Th/Ta ratios (<1.5) and Pb and U contents also indicate the absence of crustal contamination (Table 1). This interpretation is consistent with the Nd isotopic variations (Table 2). The normalized multi-element and REE patterns (Fig. 6), and ratios of incompatible elements (Table 1) suggest that once geochemical variation due to fractional crystallization is accounted for, the variation in the chemistry of the different magmatic YC rocks is a function of mantle heterogeneity. According to Allègre and Minster (1978) and Baker et al. (1997), an estimation of primary magma composition and mantle sources can be obtained using incompatible element ratios and hygromagmatophile element ratios (e.g., Ce/Y, Nb/Yb, and (Th/Ta)/(Th/Tb)). The hygromagmatophile elements are either weakly fractionated during fractional crystallization or during partial melting. Therefore, rocks originating from chemically different mantle sources will have distinct values of (Th/Ta)/(Th/Tb). The Th/Ta vs. Th/Tb ratios for the YC magmatic rocks indicate two distinct groups of samples (Table 1), supporting the conclusion that the two groups were derived from chemically distinct mantle sources. In addition, the OIB samples have high values of Ce/Y (1.40–2.84) and (La/Yb)N (4.75–9.98), whereas the MORB samples have low Ce/Y (0.17–0.60) and (La/Yb)N (0.29–2.07). Considering that Ce and La are more incompatible than Y and Yb, rock samples with the smallest degree of partial melting or derived from enriched sources are expected to exhibit high Ce/Y and (La/Yb)N ratios (Saccani et al., 2003). The MORB samples have low values of incompatible elements, such as Ta/Hf, Th/Yb, and Ta/Yb (Table 1), suggesting that the source was both infertile and extensively melted. In contrast, the OIB samples show much higher

ratios of Ta/Hf, Th/Yb, and Ta/Yb relative to MORB, and such strong enrichment in highly incompatible elements may indicate limited partial melting of a mantle source, leaving a garnet-bearing residue (Aldanmaz, 2002).

6.2. Mantle sources and melting processes

Melt modeling, which involves plotting the garnet-dependent ratio Sm/Yb against La/Sm, is another approach to investigating the nature of mantle sources (Fig. 9a; Shaw, 1970). Aldanmaz et al. (2000) suggested that when spinel lherzolite undergoes partial melting, the mantle and melt produced will have similar Sm/Yb ratios, whereas La/Sm ratios decrease with increasing degrees of partial melting. Melting of a spinel lherzolite source will create a horizontal melting trend, which lies within or close to a mantle array defined by DMM and PM compositions. In contrast, a small to moderate degree of partial melting of a garnet-lherzolite source produces a melt with significantly higher Sm/Yb ratios than the mantle source. Fig. 9a thus shows that the OIB samples plot between the melting trajectories for garnet and spinel lherzolite, even for an enriched mantle of the Western Anatolian Mantle (WAM) composition. Given that the MORB samples plot along the mantle array, this suggests their melts were generated from partial melting of a depleted component in the spinel facies mantle, though the E-MORB shows slightly higher Sm/Yb ratios than the N-MORB. Meanwhile, in the Sm/Yb vs. Sm diagram (Fig. 9b; Shaw, 1970), the data points of alkaline OIB plot around the WAM curve garnet+spinel-lherzolite melting trend, which indicates the presence of garnet residue in the source region. Conversely, the tholeiitic N-MORB samples plot near the DMM curve, indicating a low degree (3%–20%) of partial melting of a DM source in the spinel facies zone, whereas the composition of the enriched E-MORB samples can be explained by melting of a depleted mantle component with a small contribution from an enriched (e.g., OIB-type) mantle component. The mantle source and degree of partial melting of the mafic volcanic rocks can also be investigated using the Yb vs. La/Yb diagram, using the theoretical partial

melting curves for garnet and spinel lherzolite sources as proposed by Baker et al. (1997) and Rotolo et al. (2006) (Fig. 9c). Two distinct trends in the diagram are observed, corresponding to the OIB and MORB rocks of the YC. In this diagram, the OIB samples plot in a transition zone between the garnet-lherzolite and spinel-lherzolite melting curves, which implies that the magma originated from partial melting of the deeper mantle in the garnet-facies stability zone and continued to melt in the spinel facies zone, as part of a parabolic melt system. The MORB samples plot precisely along the spinel-lherzolite melting curve, indicating that they originated from a relatively shallow mantle source with spinel as a stable phase. In summary, we propose that the OIB rocks were generated from a significantly enriched-type mantle, which began to melt in the garnet field and continued melting at spinel-facies mantle depths. In contrast, the MORB rocks were generated by partial melting of a depleted MORB source in the spinel facies mantle, with the source of E-MORB samples significantly enriched by OIB-type components.

6.3. Formation of OIB and MORB rocks in the Yuejinshan Complex

The melt modeling presented above shows that the source of the OIB-type alkaline rocks was enriched in LREE relative to DMM and PM, suggesting the presence of enriched components in the mantle sources (Hofmann, 1997). In contrast, the Nd isotope ratios of the alkaline rocks are characterized by positive Nd (+6.20 to +8.61) values, indicating a source significantly depleted relative to the signature of bulk Earth, though less depleted than most N-MORB. This type of mantle source is the rule, and often these enriched components are produced by the incorporation of recycled oceanic crust with or without sediment (Hofmann and White, 1982; Hofmann, 1997), recycled metasomatized lithospheric mantle (McKenzie and O’Nions, 1995; Niu and O’Hara, 2003; Pilet et al., 2008), or melts that originate in the asthenosphere (Hirano et al., 2006). The recycled material interacts with the mantle at various depths during delamination or subduction and can be returned to the surface by positive buoyancy, mantle convection, or via deep-seated plumes rising from the

core–mantle boundary or the mantle transition zone of stagnant status (Allegre and Turcotte, 1985; Anderson, 2007; Hart, 1988; Hart et al., 1992; Hofmann, 1997; Keken et al., 2002; Weaver, 1991). The geochemical signature of the OIB samples implies no arc-related magmatism was involved. However, some arc-related magmas have been identified in the Jiamusi Massif (Bi et al., 2016, 2017; Yang et al., 2015). Thus, we conclude that the OIB-type alkaline samples were derived from a deep OIB reservoir, brought to the surface by a mantle plume that possibly formed seamounts on the ocean floor. Previous studies have suggested that MORB are chemically uniform because they are the products of decompression melting of a homogeneous reservoir on a global scale (Dupre et al., 1981; Macdougall and Lugmair, 1985). All the MORB-type samples in this study exhibit significant depletions in LREE together with positive Nd (t) (+10.2 to +10.5) values, and their chondrite-normalized REE patterns coincide with those of MORB (Fig. 6c and d). Most of the samples fall into the fields of MORB and BABB on the V-Ti/1000 diagram (Fig. 7a), and all of the samples plot within the N-MORB and E-MORB fields on tectonic discrimination diagrams (Fig. 7b–d). These MORB samples were probably derived from a depleted or slightly enriched mantle source in an oceanic spreading center or marginal basin. Considering that mafic rocks formed at mid-ocean ridge settings are unlikely to be preserved after the subduction and accretion of an oceanic plate, we favor a back-arc spreading setting to explain the formation of the MORB in this study. Geological evidence supports the existence of sediments from continental slope facies in the YC (Shao and Tang, 1995), and subduction events in the eastern Jiamusi Massif have been documented previously (Bi et al., 2016, 2017; Yang et al., 2015; Yu et al., 2013). Therefore, we propose that the OIB and MORB were derived from seamounts and back-arc lavas respectively, of the oceanic crust of the Paleo-Pacific Ocean.

6.4. Tectonic implications

The Yuejinshan Complex is located between the Jiamusi Massif and the

Nadanhada Terrane in NE China, and represents the remnants of the late Paleozoic western Paleo-Pacific Ocean, consisting of strongly deformed metaclastic rocks and metabasaltic lavas. In this study, the Yuejinshan mafic volcanic rocks are found to have a composition that ranges from depleted/enriched MORB to OIB-types, and these are found juxtaposed with each other in the YC. Spatially, the OIB-type alkaline lavas are distributed to the west of the MORB-type tholeiites. Unfortunately, insufficient zircons were available to ascertain a date for the metabasaltic rocks. The emplacement time and mechanisms of the YC cannot be constrained based on geological evidence from the present study. However, several authors have suggested that the Yuejinshan Complex was formed in a back-arc basin (Guo, 2016; Zhang et al., 2008) and was emplaced onto the eastern margin of the Jiamusi Massif during the early Jurassic (Guo, 2016). Other authors have argued that the YC, which is part of the Nadanhada Terrane, represents the first stage of an accretion complex created by subduction–accretion of the Paleo-Pacific plate (Tian, 2006; Zhang et al., 1997; Zhou et al., 2014). Zhou et al. (2014) further point out that the Yuejinshan Complex was accreted between 210 and 180 Ma, although no precise age data were provided. The timing of accretion is based on Rb–Sr ages for mica schists, which suggest that metamorphism occurred at 188 ± 4 Ma (Yang et al., 1998). Moreover, the metamorphism of the Heilongjiang Complex in the western part of the Jiamusi Massif took place between 210 and 180 Ma and was related to the onset of subduction of the Paleo-Pacific plate (Zhou et al., 2009, 2014). The Heilongjiang Complex, located in the western margin of the Jiamusi Massif, was probably formed part of an ocean between the Jiamusi Massif and Songliao Terrane during the Permian. U–Pb zircon ages of 258 ± 2 Ma, 259 ± 4 Ma (Zhou et al., 2009), 275 ± 2 Ma (Zhu et al., 2015), and 281 ± 3 Ma (Ge et al., 2016) provide a limit to the protolith ages of the Heilongjiang Complex blueschists. In addition, abundant granites and gabbros of Permian–early Triassic (275–245 Ma) age are found in the western Jiamusi Massif. Elemental and isotopic geochemical studies indicate the granites and gabbros were related to subduction of the Mudanjiang oceanic plate beneath the western Jiamusi Massif (Dong et al., 2017; Wu et al., 2001; Yang et al.,

2016). Furthermore, in the eastern Jiamusi Massif the granitoids and gabbros are considered to be subduction-related magmatic intrusions, and the LA–ICP–MS U–Pb zircon ages of 302–260 Ma constrains the timing of the intrusions (Bi et al., 2014b, 2016; Yang et al., 2015; Yu et al., 2013). The timing of subduction is also constrained by U–Pb zircon ages of 291–263 Ma reported for bimodal volcanic rocks (Bi et al., 2017; Meng et al., 2008). These volcanic rocks are located to the west of the YC and exhibit similarities to a mature volcanic arc, indicating the late Carboniferous to middle Permian evolution of the Jiamusi Massif involved the convergence of the Paleo-Pacific Ocean (Bi et al., 2016, 2017; Yang et al., 2015). Considering the ages of igneous rock from the eastern Jiamusi Massif and the evolution of Heilongjiang Complex in the western Jiamusi Massif, we propose that the YC was accreted along the eastern margin of Jiamusi Massif during the late Carboniferous to middle Permian, caused by westward subduction of the Paleo-Pacific Ocean. A schematic tectono-magmatic model is presented in Fig. 10, showing the possible tectonic settings for the evolution of MORB and OIB mafic volcanics within the YC. We suggest that the Yuejinshan Complex formed above the oceanic plate of the Paleo-Pacific Ocean before 305 Ma. The YC includes dismembered remnants of oceanic crust generated in a back-arc spreading setting, and ocean crust generated in an intraplate seamount setting, above a mantle plume (Fig. 10a). Subsequently, the Yuejinshan Complex was accreted onto the active margin of the eastern Jiamusi Massif during late Carboniferous–middle Permian subduction of the Paleo-Pacific Ocean crust (Fig. 10b). In addition to the YC in northeastern China, late Paleozoic accretionary complexes related to subduction of the Paleo-Pacific Ocean crust also formed in the Russian Far East and the Japanese Islands; e.g., the Akiyoshi accretionary complex in SW Japan and the Khabarovsk accretionary complex in western Sikhote–Alin (Isozaki et al., 1990; Kanmera and Sano, 1991).

7. Conclusions

Based on the petrological and geochemical characteristics of mafic volcanic

rocks of the Yuejinshan Complex, northeastern China, we arrive at the following main conclusions. (1) The mafic volcanics of the YC exhibit a range of geochemical variations and are identified as tholeiitic mid-ocean ridge basalts (MORB) and alkaline ocean island basalts (OIB). (2) Whole-rock trace element geochemistry shows that the MORB-type rocks range in composition from light rare earth element (LREE)-depleted varieties (N-MORB; (La/Sm)N < 1) to transitional MORB, to LREE enriched rocks (E-MORB; (La/Sm)N > 1). The OIB-type alkaline rocks are characterized by significant enrichments in LILE, HFSE and L-MREE, and a slight depletion in HREE, relative to MORB. (3) REE petrogenetic modeling shows that the tholeiitic basaltic rocks were generated from partial melting of a DMM source in the spinel facies mantle. In contrast, the alkaline basalts were generated from an enriched OIB-type mantle source that originated in the stability field of garnet and continued to melt in the stability field of spinel. (4) The current juxtaposition of MORB and OIB in the YC of the eastern Jiamusi Massif, coupled with other geological evidence, indicates that the MORB lavas were erupted in a back-arc basin setting and the OIB erupted at an intraplate-seamount setting. Subsequently, the oceanic lithosphere accreted along the eastern Jiamusi Massif due to the progressive advance of the Paleo-Pacific Ocean plate, most likely during the late Carboniferous to middle Permian.

Acknowledgments

We thank Xi-Rong Liang for assistance with Sr and Nd isotope analyses. We also thank the Analytical Laboratory of the Beijing Research Institute of Uranium Geology and Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. This research was supported by the National Natural Science Foundation of China (Grants 41472050 and 41330206). The final version of this paper benefited from perceptive comments by Fu-Yuan Wu (Guest Editor), Simon A. Wilde and an anonymous

reviewer.

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Figure captions Fig. 1. Location of blueschist and OIB-hosting accretionary complexes in Central and East Asia (modified from Safonova, 2016). Fig. 2. (a) Geological sketch map of NE China and adjacent areas (modified from Zhou et al., 2013). F1 = Yitong–Yilan Fault; F2 = Jiayin–Mudanjiang Fault; F3 = Dunhua–Mishan Fault; F4 = Hegenshan–Heihe Fault; F5 = Xinlin–Xiguitu Fault; F6 = Solonker–Xar Moron–Changchun zone. (b) Simplified geological map of the Jiamusi Massif and Nadanhada Terrane (modified from Bi et al., 2017). Fig. 3. Detailed geological map and sample locations in the Qindeli and Yuejinshan areas. Fig. 4. Representative field photos and micrograph photos of samples analyzed in this study. (a) Field photograph of samples in the Qindeli area. (b, c) Outcrop of the fragmented metabasaltic rocks. (d) Sample 10GW182 from Jianshan containing actinolite grains. (e) Sample 12GW070 from Qindeli showing folding deformation. (f) Sample 15GW424 from Dongshan displaying plagioclase grains preserved within a matrix composed of chlorite and carbonate minerals. Fig. 5. (a) Classification of the metabasaltic rocks using the SiO2 vs. Nb/Y diagram of Winchester and Floyd (1977), and (b) the FeO*+TiO2–Al2O3–MgO ternary diagram, after Jensen (1976). Fig. 6. Chondrite-normalized REE patterns (a and c) and N-MORB normalized incompatible element patterns (b and d) for metabasaltic rocks from the Yuejinshan

Complex. Normalizing values are from Sun and McDonough (1989). Fig. 7. Tectonic discrimination diagrams for the metabasaltic rocks from the Yuejinshan Complex. (a) V-Ti/1000 diagram (after Shervais, 1982); (b) Th/Yb vs. Nb/Yb diagram (the field of the MORB-OIB mantle array is from Pearce, 2008); (c) 2Nb–Zr/4–Y diagram (after Meschede, 1986); and (d) Y/15–La/10–Nb/8 diagram (after Cabanis and Lecolle, 1989). Fig. 8. Nd vs. ISr diagram for metabasaltic rocks from the Yuejinshan Complex (after DePaolo and Wasserburg, 1977). Fig. 9. (a) Sm/Yb vs. La/Sm and (b) Sm/Yb vs. Sm diagrams showing melt curves obtained using the non-modal batch melting equations of Shaw (1970). Melt curves are drawn for spinel lherzolite (with mode and melt mode after Kinzler, 1997) and for garnet lherzolite (with mode and melt mode after Walter, 1998). Mineral/matrix partition coefficients and DMM are from McKenzie and O’Nions (1991, 1995). PM, N-MORB, and E-MORB compositions are from Sun and McDonough (1989). WAM represents the Western Anatolian Mantle, defined by extrapolating the best-fit melting trajectories drawn for the Western Anatolian primitive alkaline rocks. The heavy line represents the mantle array defined using DMM and PM compositions. Dashed and solid lines are the melting trends from DMM and WAM, respectively. Purple squares on each line correspond to the degree of partial melting for a given mantle source. (c) Yb vs. La/Yb diagram for mafic volcanics of the YC. Modal melting curves of garnet-lherzolite and spinel-lherzolite sources (dotted arrows) are from Baker et al. (1997), and the scale for percentage melting is from Rotolo et al. (2006). MORB and OIB fields, from Baker et al. (1997) and El-Rahman et al. (2009), are shown for comparison. Fig. 10. Tectono-magmatic evolution of the mafic volcanics of the Yuejinshan Complex, NE China. (a) Early phases of back-arc basin spreading and plume-related volcanism; and (b) subduction of oceanic plate at ca. 305~260 Ma.

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4

5

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5

3

0

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1

9

8

1

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0

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9

5

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9

5

0

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2

9

4

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T

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(

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u

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U

0.

1.

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a

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3

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1

5

C 2

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Y 1

8

1

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1

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3

0

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0

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2

2

1

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T

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S m ) N

( L

b

0.

0.

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4

6

6

5

6

6

8

2

5

9

8

0

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2

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8

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7

9

0

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0

8

6

7

3

8

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9

) N

N b / Y b

a T h / T b T a / H f T h / Y b T a / Y b E u / E u

2

2

2

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1

1

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6

0

2

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6

3

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6

9

1

7

5

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7

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8

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9

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0

1

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1

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1

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2

2

2

8

2

2

1

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0

0

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7

0

1

1

1

1

1

1

1

5

4

2

3

0

3

3

2

4

8

3

2

1

0

1

1

0

1

1

0

1

1

1

1

1

1

.

.

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.

.

9

1

1

9

0

0

9

1

0

1

0

0

0

3

1

6

7

2

3

5

1

4

1

6

3

5

*

Table 2 Sr-Nd isotopic data for the metavolcanic rocks from the Yuejinshan accretionary complex, NE China Type I

Type IIa

Jianshan

0.701222

4.47

7.32

0.000007

0.701718

5.74

8.61

0.000009

0.700666

4.21

6.22

0.512829

0.000007

0.701759

3.73

6.40

0.125921

0.512829

0.000008

0.704505

3.73

6.62

0.000017

0.129525

0.512815

0.000007

0.704594

3.45

6.20

0.705425

0.000016

0.130114

0.512842

0.000007

0.705362

3.98

6.71

0.026157

0.707226

0.000014

0.230691

0.513247

0.000006

0.707107

11.9

10.5

423

0.024279

0.706515

0.000017

0.206584

0.513179

0.000008

0.706404

10.6

10.2

363

0.040804

0.706504

0.000020

0.226629

0.513223

0.000009

0.706318

11.4

10.2

Nd

Rb

Sr

87

11GW075

320

8.98

42.7

43.9

63.7

1.994446

0.710305

0.000018

0.127148

0.512867

0.000007

11GW076

320

9.86

47.1

65.8

105

1.813510

0.709977

0.000018

0.126568

0.512932

13GW176

320

14.1

57.7

71.6

92.4

2.242656

0.710880

0.000018

0.147742

0.512854

13GW179

320

11.1

51.1

82.6

87.6

2.729846

0.714191

0.000026

0.131329

10GW188

320

6.04

29

3.99

330

0.034972

0.704664

0.000014

13GW002

320

6.32

29.5

19.2

690

0.080486

0.704961

13GW003

320

7.64

35.5

4.21

885

0.013760

13HHL-4-1

320

3.14

8.23

2.64

292

13HHL-4-2

320

4.51

13.2

3.55

13HHL-4-4

320

5.06

13.5

5.12

Qindeli

Nanlingongduan

Nd(t)

Sm

Sample

Jiangbian

Nd(0)

t(Ma)

Location

Rb/86Sr

87

Sr/86Sr

2s

147

Sm/144Nd

143

Nd/144Nd

2

(87Sr/86Sr)i

1. Tholeiitic MORB and alkaline OIB coexist in the Yuejinshan Complex, NE China. 2. Tholeiitic basaltic rocks were likely derived from a depleted MORB source in the spinel facies mantle. 3. Alkaline rocks were derived from an enriched OIB-type mantle source in the garnet facies and continued melting to spinel facies mantle depths. 4. Back-arc basin spreading associated with plume-related volcanism is proposed to explain their generation.