Zircon U-Pb chronology, geochemistry and geological significance of the Tongjiang-Fuyuan Mesozoic magmatic rocks, NE China

Abstract

Typical ophiolitic rock assemblages such as siliciclastic rocks, basalts and gabbros, together with the subduction-related intermediate-acidic intrusive rocks, are newly discovered in the Tongjiang-Fuyuan area of the Heilongjiang Provence, NE China. To determine the formation age and genesis of the mafic rocks (basalts and gabbros) and intermediate-acidic intrusive rocks (granodiorites) in the area, as well as their geodynamic settings, the whole-rock geochemical analysis and zircon LA-ICP-MS U-Pb dating were carried out. Zircon U-Pb results suggest that the granodiorites are 93–95 Ma and gabbro is 95 Ma, respectively. Geochemical results show that the gabbros and basalts exhibit characteristics of ocean island basalt (OIB) affinity and are typically related to having originated from mantle plumes. While the granodiorites show the nature of the island-arc magmatic rocks and may originate from the lower crust. Based on the coeval igneous rock associations and regional tectonic evolution, we conclude that the late Cretaceous magmatic rocks in the Tongjiang-Fuyuan area are the product of continuous subduction of the Palaeo-Pacific plate and reflect the subduction rollback process of the Palaeo-Pacific plate.

1. Introduction

Ophiolites, as fragments of ancient oceanic lithosphere (e.g. Dewey & Bird, 1971; Coleman, 1977), play irreplaceable roles in the recognition and reconstruction of the evolution history of an ancient ocean, including the opening, closure, development of subduction systems and the consequent orogeny (Dilek, 2003; Dilek et al., 2007; Dilek & Furnes, 2011). Investigation of the formation and emplacement age of an ophiolite can unravel the process of accretionary orogenesis (Xiao et al., 2009a, 2013). Furthermore, the ophiolite or ophiolitic mélange is the most basic tectonic member in the subducted wedge of an orogenic belt, which is not only an important marker of the Palaeo-plate boundary but also important for understanding the evolution process of the orogenic belt (Jian et al., 2003; Zhang et al., 2003; Shi, 2005; Dilek & Furnes, 2011; Furnes & Dilek, 2017).

Northeast China (NE China) is located in the eastern part of the Central Asian Orogenic Belt (CAOB), which underwent the complicated subduction-accretionary orogenic processes (Xiao et al., 2009b; Zhang et al., 2013, 2015; Chen et al., 2015; Ge et al., 2015; Wang et al., 2015). As we all know, the NE China is tectonically composed of four micro-blocks and an accretionary terrane, i.e. Erguna Block (EB), Xing’an Block (XB), Songliao-Xilinhot Block (SXB), Jiamusi Block (JB) and the Nadanhada Terrane (NT). A large number of ophiolites or ophiolitic complexes are widely distributed in NE China, significantly marking the suture zones among the above-mentioned blocks and terrane. From west to east, they are the Xinlin-Xiguitu, Heihe-Nenjiang-Hegenshan, Mudanjiang-Yilan and Yuejinshan belts (Fig. 1), involving the closure of the ancient oceans of Neoproterozoic-Late Cambrian Xinlin-Xiguitu branch ocean, the Early Cambrian-Late Carboniferous Nenjiang branch ocean, Neoproterozoic Heilongjiang branch ocean, the Late Permian-Middle Jurassic Mudanjiang branch ocean and the subduction of the Mesozoic Palaeo-Pacific ocean, respectively (Zhang et al., 2008; Feng et al., 2019; Liu et al., 2019).

Figure 1. Tectonic divisions of the NE China (after Liu et al., 2017b).

During the Mesozoic, NE China was affected by the closure of the Mongolian-Okhotsk Sea and the subduction of the Palaeo-Pacific plate, resulting in large-scale tectonic-magmatic activities (e.g. Wu et al., 2000; Meng et al., 2014; Feng et al., 2019; Liu et al., 2019; Li et al., 2020). Previous studies on Mesozoic volcanic rocks in NE China show that the Mesozoic accretive complexes, the arc magmatic belts of JB and the eastern margin of SXB in NE China are distributed in the south-north direction (e.g. Xu et al., 2013; Wilde, 2015; Liu et al., 2017b, 2019, 2021; Han et al., 2022). These accretive complexes formed in the Late Triassic to Early Cretaceous and showed a trend of becoming younger from west to east (Sun, 2013; Sun et al., 2014; Li et al., 2020). The above facts indicate that the Mesozoic accretive complex in NE China is closely related to the western subduction of the Palaeo-Pacific plate (Li et al., 2020; Han et al., 2022).

To the eastmost of NE China, the typical ophiolites (ophiolitic complexes) are discovered in the Raohe and Yuejinshan areas from the NT. Zhang et al. (1997) found that the Yuejinshan Complex is a tectonic mélange with the block-in-matrix texture and proposed that the original rocks are N-MORB-type basalts, which are typical ophiolites. Meanwhile, the concept of the Raohe ophiolite was first proposed by Li (1980) and further has been accepted by many scholars (Kojima & Mizutani., 1987; Kojima, 1989; Zhang et al.,1989; Kang, et al., 1990; Mizutani & Kojima, 1992). However, typical mantle peridotites do not develop in ophiolites in the Raohe area (Zhang et al., 2000), and the geochemical characteristics of mafic-ultramafic complexes are quite different from typical ophiolites, so some scholars regard it as the OIB-type Complex (Zhang et al., 1998, 2003; Zhang and Zhou, 2001).

In recent years, the studies on the NT have pointed out that the Yuejinshan Complex and the Raohe Complex are the direct products of the long-term subduction and accretion of the Palaeo-Pacific plate beneath the Eurasian continent (Zeng et al., 2018). However, the research on the evolution process of the NT is still controversial; especially, there is a lack of evidence for the age of the Late Cretaceous. Yu et al. (2013) discovered the Late Cretaceous granites in the Tongjiang-Fuyuan area in the northern part of the NT, which is different from the previous research that the granites developed in the late Indosinian belt in Raohe (HBGMR, 1993). According to field surveys and whole-rock geochemical analysis, it is found that the Late Cretaceous granites in the Tongjiang-Fuyuan area were formed in the active continental margin tectonic environment generated by the subduction of the Palaeo-Pacific plate to the East Asian continent (Yu et al., 2013). Significantly, the typical ophiolites such as siliceous rocks, basalts and gabbros are also developed in the Tongjiang-Fuyuan area; however, they have not been systematically studied.

This study therefore focuses on the ophiolitic rock assemblages such as siliciclastic rocks, basaltic rocks and gabbros together with the granodiorite exposed in the Tongjiang-Fuyuan area in the northern part of the NT, NE China and carried out the systematic studies on field investigations, geochronology and geochemistry, to further reveal the subduction-accretionary process of the Palaeo-Pacific plate.

2. Geological setting and sampling

2.a Regional geology

The NE China comprises multiple micro-continental blocks or terranes, namely the EB, XB, SXB and JB, as well as the easternmost Nadanhada (Tang et al., 1995; Li, 2006; Zhou & Wilde, 2013), and it is tectonically located in the triangular zone of the Palaeo-Asian Ocean, Mongolia-Okhotsk Ocean and the Palaeo-Pacific plate tectonic domains (Tang et al., 1995; Li, 2006; Zhou et al., 2009, 2014) (Fig. 1). The EB is distributed in a northeastward direction and is connected with the XB on the southeast side, and the magmatic rock age of EB is mainly Neoproterozoic, Early Palaeozoic, Late Palaeozoic and Mesozoic. (Liu et al., 2008; Wu et al., 2011; Zhang et al., 2011; Zhou et al., 2014; Dong et al., 2019; Li et al., 2020; Han et al., 2022). The XB is adjacent to the EB and SXB and is mainly composed of Palaeozoic-Mesozoic magmatic rocks and related volcanic ⁃ sedimentary layers (Zhang et al., 2011; Zhou et al., 2014; Dong et al., 2019; Li et al., 2020; Han et al., 2022). The SXB is located between the XB and JB, the magmatic rocks are mainly formed in Neoproterozoic, Early Palaeozoic, Late Palaeozoic and Mesozoic (Zhang et al., 2011; Zhou et al., 2014; Dong et al., 2019; Li et al., 2020; Han et al., 2022). The JB is adjacent to the SXB and NT, the magmatic rocks are dominated by granitic rocks, and the ages are mainly early Palaeozoic Cambrian, Late Palaeozoic Permian, Mesozoic Triassic and Cretaceous (Zhang et al., 2011; Zhou et al., 2014; Dong et al., 2019; Li et al., 2020; Han et al., 2022). The NT is located in the easternmost part of the NE China and is mainly composed of Yuejinshan complex, Raohe complex and Early Cretaceous magmatic rocks (Zhou et al., 2014; Tang et al., 2018; Li et al., 2020; Han et al., 2022).

In recent years, the key and difficulty to the study on the tectonics of NE China, as well as the dynamic mechanism of the major tectonic domains, lies in the superposition and transformation of the subduction-accretion of the Palaeo-Pacific plate, and the NT in the easternmost part of NE China happens to be the subduction-accretionary process of Palaeo-Pacific plate tectonic domain (e.g. Kojima & Mizutani, 1987; Mizutani & Kojima, 1992; Zhou et al., 2014; Tang et al., 2018; Han et al., 2022).

The NT preserves relatively intact marine sedimentary strata. A stratigraphic palaeontological comparison suggests that the NT in the NE China, the Sikhote-Alin terrane in the Russian Far East and the Menon-Tamba terrane in Japan formed a super Jurassic accretionary terrane on the eastern edge of Eurasia continent before the opening of the Sea of Japan (Mizutani et al. 1989; Kojima, 1989). In the past half-century, many previous researchers provided different understandings of the tectonic properties of the NT. Wang (1959) regarded the area as a Mesozoic trough folded zone. Li et al. (1979) considered the area to be a Late Palaeozoic trough fold zone based on fossils from the Carboniferous to Permian in the tuffs. Ren et al. (1980) suggested that the area is a Mesozoic geosyncline on a Variscan folded basement. In addition, since the Carboniferous gill fossils in the limestone of the NT have typical Tethys tectonic domain fossil assemblages (Zhang et al., 1989), Palaeomagnetic evidence and radiolarian fossil comparisons also show that the NT has experienced a long period of migration history at the Mesozoic (Mizutani et al., 1989; Ren et al., 2015). Therefore, some scholars believe that the NT is an extraneous terrane from low latitudes (Mizutani et al., 1989; Wu et al., 2011; Li et al., 1979; Zhang & Ma, 2010). More recently, with the gradual maturation of plate tectonic theory and the identification and study of sedimentary rocks, some scholars have also suggested that the NT is an accretionary terrane formed by the subduction of the Palaeo-Pacific plate beneath the JB (Zhou et al., 2014; Sun et al., 2015b; Liu et al., 2017b, 2021; Li et al., 2020).

According to previous studies, the NT is divided into two main tectonic units, e.g. Yuejinshan Complex and Raohe Complex (e.g. Zhou et al., 2014, 2018; Bi et al., 2015, 2016, 2017a, 2017b; Wilde, 2015; Sun et al., 2015a; Zeng et al., 2019; Xu et al., 2020; Zhang et al., 2020; Han et al., 2022). The Yuejinshan Complex is mainly composed of siliceous rocks, metamorphic clastic rocks and mafic-ultramafic rocks. Metamorphic clastic rocks include quartz schist, marble and mica schist, suffering the greenschist-facies metamorphism (Zhang et al., 1997; Yang et al., 1998; Bi et al., 2016, 2017a, 2017b; Ge et al., 2016; Zhou et al., 2018; Han et al., 2022). The mafic-ultramafic rocks show a typical ophiolite sequence, which is composed of metabasalt, gabbro and clinopyxroxenite (Zhou et al., 2014; Bi et al., 2016, 2017a, 2017b; Ge et al., 2016; Zhou et al., 2018; Han et al.,2022). Due to the destruction of tectonic deformation, the Yuejinshan Complex shows a tectonic mélange with metamorphic clastic rocks as the matrix and mafic-ultramafic rocks as blocks; thus, the Yuejinshan Complex is considered as a set of ophiolitic mélanges (Zhang et al., 1997; Zhou et al., 2014; Dong et al., 2019; Han et al., 2022). The formation of Yuejinshan Complex has been the subject of debate, with some studies suggesting that it was formed in the middle of the Palaeozoic, while others suggest that it may have formed from the Late Triassic to the Early Jurassic (HBGMR, 1993; Zhang et al., 1997; Yang et al., 1998). Recent studies have shown that the Yuejinshan Complex formed at around 210–180 Ma and that it represents the first stage of an accretion complex created by subduction-accretion of the Palaeo-Pacific plate (Zhou et al., 2014; Yang et al., 2015; Bi et al., 2016, 2017a, 2017b; Cao et al., 2019; Han et al., 2022).

The Raohe Complex is mainly composed of tuffs, siliceous rocks, mafic-ultramafic rocks and clastic rocks and the clastic rocks mainly contain sandstone and mudstone, which form the matrix of the Raohe Complex (Zhou et al., 2014; Bi et al., 2016, 2017a, 2017b; Ge et al., 2016; Zhou et al., 2017; Han et al., 2022). The Raohe Complex is similar to typical ophiolitic suites in that they are stratified by oceanic plate sediments, pillow basalt, stacked crystal gabbro and ultramafic rocks, which led to the conclusion that the Raohe Complexes are a set of tectonic mélanges associated with the subduction of the oceanic plate (Zhu et al., 2015; Zeng et al., 2019; Zhang et al., 2020). The formation ages of basalt and gabbro in the Raohe-Dadai area are 166 ± 2 Ma and 214 ± 5 Ma, respectively (Zhou et al., 2014; Han et al., 2022), and the sedimentary lower limit ages of silty mudstone and sandstone samples are 167 ± 3 Ma and 133 ± 4 Ma (Zhou et al., 2014; Sun et al., 2015a; Zeng et al., 2019; Han et al., 2022). Additionally, the stapled granite intruded into the Raohe Complex was mainly formed at 126–110 Ma, limiting the final emplacement age of the Raohe Complex to 133–126 Ma (Cheng et al., 2006; Zhou et al., 2014; Zeng et al., 2019; Han et al., 2022).

Previous studies on magmatic rocks in NE China showed that the formation age of magmatic rocks was distributed from the Early-Middle Jurassic to the Late Cretaceous (Wu et al., 2011; Zhang and Mizutani, 2004, Cheng et al., 2006; 2008; Wang et al., 2009; Wilde et al., 2010; Yu et al., 2013; Ge et al., 2015; Zeng et al., 2018, 2019; Li et al., 2020). Late Cretaceous granites were firstly discovered in the Tongjiang-Fuyuan area in the north of NT (Yu et al., 2013), and these granites were considered to be formed in the tectonic setting related to the subduction of the Palaeo-Pacific plate beneath the East Asian continent.

2.b Sample locations and simple description

The investigated Tongjiang-Fuyuan area is located at the junction of the JB and the NT (HBGMR, 1993; Yu et al., 2013; Wang et al., 2017), which is heavily covered and has few exposed rock masses. Through field investigation, it is found that the rocks exposed in the area are mainly composed of siliceous rock, basalt, gabbro and granodiorite, looking like the dismembered ophiolitic suites (Fig. 2).

Figure 2. Geological map of the study area of Tongjiang-Fuyuan (modified after HBGMR, 1993; Yu et al., 2013).

Our petrological investigations suggest that the gabbro, basalt and siliceous rocks developed in the Tongjiang-Fuyuan area are contacted by the top-to-west thrusting faults, indicating a near EW-trending compression (Fig. 3a). The gabbros are greyish green or greyish black with various weathering degree and have the massive or lensed structure (Fig. 3b). The basalts occasionally show the pillow-like structure (Fig. 3c). The siliceous rocks (cherts) are strongly deformed, generally forming the tight folds (Fig. 3d). The granodiorites are grey or white in colour with coarse-grained phanerocrystalline texture and massive structure (Fig. 3e, f).

Figure 3. Field photographs of the basalt, gabbro and granodiorite collected from the Tongjiang-Fuyuan area, NE China. (a) A thrust fault developed between the basalts, gabbro and siliceous rock. (b) Partially weathered gabbro. (c) Basalts with the pillow-like structure. (d) A strongly deformed siliceous rock (cherts) forming the tight folds. € (f) coarse-grained granodiorite.

In the study, we collected a total of 9 samples, including 3 mafic rocks and 6 intermediate-acidic intrusive rocks. The detailed sampling locations and petrographic features are available in Figs. 3 and 4 and Table 1. The detailed descriptions for the representative samples are as follows.

Figure 4. Photomicrographs of the basalt, gabbro and granodiorite rock samples collected from the Tongjiang-Fuyuan area, NE China. (a), Basalt (17TJ2). (b), Gabbro (17TJ5-1). (c), Fine-grained granodiorites (17FY3). (d), Coarse-grained granodiorites (17FY9). Pl = plagioclase; Cpx = clinopyroxene; Bt = biotite; Amp = amphibolite.

Table 1. Mineral association and petrographic characteristics of the investigated Tongjiang-Fuyuan magmatic rocks

Basalts (17TJ2) exhibit porphyritic textures with phenocrysts of clinopyroxene (70%) and minor plagioclase (25%). Clinopyroxene phenocrysts are 100–150 µm in size and display irregularity and obvious fractures. Minor plagioclase phenocrysts are 90–120 µm in size and display polysynthetic twinning (Fig. 4a). The matrix is composed of plagioclase microlites, granular clinopyroxene, chlorite and opaque minerals (Fe-Ti oxide). Many clinopyroxenes are partially to completely replaced by chlorite and opaque minerals (Fig. 4a).

Gabbros (17TJ5-1) display a diabase structure and consist of orthopyroxene (50%), plagioclase (45%), chlorite and Fe-Ti oxides (opaque minerals). Plagioclases are relatively subhedral and vary from 200 to 400 µm in size. The clinopyroxenes are subhedral and partly replaced by chlorite (Fig. 4b).

Granodiorites (17FY3) display fine-grained granitic textures, and have the mineral assemblage of plagioclase (∼50%) and quartz (∼35%), with a minor amount of hornblende and biotite. Plagioclases are subhedral and vary from 150 to 300 µm. Quartz is subhedral to anhedral and 70–110 µm in size (Fig. 4c).

Granodiorites (17FY9) display coarse-grained granitic textures and are composed mainly of plagioclase (∼40%), quartz (∼35%), hornblende (20%) and biotite (15%). Plagioclases are euhedral and vary from 150 to 200 µm. Quartz is subhedral to anhedral and 200–400 µm in size (Fig. 4d).

3. Analytical methods

3.a Zircon U-Pb dating

Zircons were separated from crushed investigated samples using conventional heavy liquid and magnetic techniques and purified by handpicking under a binocular microscope at the Yuneng Mineral Separation Company in Hebei Province, China. The selected zircons were adhered to the surface of the epoxy resin and polished, and exposed to the surface of the zircon nuclear portion to make a target. The laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb analyses were completed using an Agilent 7500a ICP-MS system equipped with a 193-nm laser at the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources, Jilin University.

To achieve the most accurate experimental results, the NIST SRM610, a reference material for synthetic silicate glass developed by the National Institute of Standards and Technology, was used to optimize the instrument. Harvard International Standard zircon 91500 zircon was selected as the experimental external standard to calibrate for elemental and isotopic fractionation. Isotope ratio data processing was performed by GLITTER (Ver. 4.0 Macquarie University). To reduce the effect of ordinary Pb on the test results, the experimental data were calibrated by Andersen (2002) to correct the isotope ratios. The age calculation was performed using ISOPLOT 4.0.

3.b Bulk rock major and trace elements analyses

After trimming off weathered surfaces, the selected samples were crushed in an agate mill and sifted into fine powders (<200 mesh). Geochemical analyses were conducted at the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources, Jilin University. Major element compositions of bulk rock samples were determined using XRF, with analytical uncertainties ranging from 2% to 3%. Trace element concentrations were determined using ICP-MS (Agilent 7500a) after acid digestion of samples in Teflon bombs and dilution in 2% HNO3 in the same laboratory. The accuracy is generally better than 5% for trace and rare earth elements (REE).

4. Analytical results

4.a Zircon LA-ICP-MS U-Pb age

4.a.1 Sample 17TJ3 (Granodiorite)

Zircon grains separated from the granodiorite are colourless, euhedral, or broken grains with varying long axial lengths (70–120 μm) and an axial ratio of ca. 1:1 to 2:1 (Fig. 5a). CL imaging reveals that most grains have fine oscillatory zones. Zircons have high U (118–884 ppm) and Th (80–570 ppm) with Th/U ratios of 0.37–0.97 (Table S1), together with the oscillatory zoning shown in the CL images, significantly indicating that they were magmatic zircon. 17 zircons yield apparent ²⁰⁶Pb/²³⁸U ages of 90–96 Ma, giving a weighted mean of 93 ± 2 Ma (MSWD = 0.05) (Fig. 5a).

Figure 5. Zircon U-Pb age concordia diagram and representative cathodoluminescence (CL) images of zircons from the Mesozoic magmatic rocks in the Tongjiang-Fuyuan area, NE China.

4.a.2 Sample 17TJ5-1(Gabbro)

Zircons selected from the gabbro are colourless, euhedral to subhedral grains and 50–100 μm in length, with an aspect ratio of 1:1 to 3:1 (Fig. 5b). CL imaging reveals that most grains are fairly dark with weak, banded zones, representing characteristic of mafic igneous origin (Baines et al., 2009; Grimes et al., 2009; Koglin et al., 2009). Zircons have high U (266–1182 ppm) and Th (87–757 ppm) with Th/U ratios of 0.37–0.59, indicating that they are magmatic origin (Table S1). 16 zircons yield apparent ²⁰⁶Pb/²³⁸U ages of 93–97 Ma, giving a weighted mean of 95 ± 2 Ma (MSWD = 0.27) (Fig. 5b).

4.a.3 Sample 17FY3 (Granodiorite)

Zircons from granodiorite are colourless, transparent and subhedral to euhedral in shape. Their sizes range from 50 to 180 μm in length, with aspect ratios of 1.5:1 to 2:1 (Fig. 5c). CL images commonly show the oscillatory zonings for the investigated zircon grains. A total of 17 analyses were made on 17 zircons, they have U and Th contents and Th/U ratios ranging from 279 to 799 ppm, 124 to 270 ppm, and 0.25 to 0.78, respectively (Table S1). 17 zircons yield apparent ²⁰⁶Pb/²³⁸U ages of 90–98 Ma, giving a weighted mean of 95 ± 2 Ma (MSWD = 0.49) (Fig. 5c).

4.a.4 Sample 17FY9 (Granodiorite)

Zircons picked out from granodiorite are colourless, subhedral or broken grains. Their grain sizes range from 70 to 180 μm in length, with aspect ratios of 2:1 to 3:1 (Fig. 5d). CL image reveals that most grains have well-developed oscillatory zones, which is suggestive of a magmatic origin. A total of 22 analyses were made on 22 zircons, the zircons have U and Th contents and Th/U ratios in the range of 235–853 ppm, 66–357 ppm and 0.29–0.79 ppm, respectively (Table S1). 22 zircons show the apparent ²⁰⁶Pb/²³⁸U ages ranging from 90 Ma to 98 Ma and a weighted mean of 95 ± 1 Ma (MSWD = 0.85) (Fig. 5d).

4.b Bulk rock major and trace elements

4.b.1 Major and trace elements in mafic rocks (basalts and gabbros)

Total 2 basaltic and 2 gabbroic samples were selected from the Tongjiang-Fuyuan area for whole-rock major and trace element analysis (Table S2).

The basalts have SiO₂ contents ranging from 42.79 to 49.33 wt%, total FeO from 10.50 to 11.19 wt%, MgO from 3.68 to 19.72 wt% and TiO₂ from 1.35 to 1.67 wt%, while the gabbros are characterized by SiO₂ (46.49–47.77 wt%), FeO (12.17–12.29 wt%), MgO (12.59–13.07 wt%) and TiO₂ (2.06–2.26 wt%) (Table S1). In the MgO-SiO₂ diagram (Fig. 6a), the basalts and gabbros plot in the basalt and the peridotite fields, respectively. In the Zr/TiO₂-Nb/Y diagram (Fig. 6b), all the Tongjiang-Fuyuan basalts and gabbros and previously documented Raohe pillow basalts plot into the alkali basalt field. By contrast, the Tongjiang-Fuyuan gabbros have lower Al₂O₃ (11.25–11.45 wt %), higher Fe₂O₃ ^T (13.55–13.56 wt%) and lower Mg# [100Mg/ (Mg + Fe²⁺) = 64–65] than those of the Tongjiang-Fuyuan basalts (Al₂O₃ = 11.25–11.45 wt%, Fe₂O₃ ^T = 11.67–12.44 wt% and Mg# = 70–75). In the K₂O-SiO₂ diagram (Fig. 6c), the Tongjiang-Fuyuan gabbros and basalts fall into the tholeiite series and are similar to the Raohe pillow basalts in the geochemical characteristics.

Figure 6. Classification diagrams of the intermediate-acidic intrusive rocks in the Tongjiang-Fuyuan area, NE China, MgO-SiO₂ (a; after Le Bas M J, 2000), Zr/TiO₂-Nb/Y (b; after Winchester and Floyd, 1976), K₂O-SiO₂ (c; after Maniar and Piccoli, 1989), A/NK-A/CNK (d; after Peccerillo & Taylor, 1976), Na₂O+K₂O-SiO₂ (e; after Irvine and Barragar, 1971), K₂O-SiO₂ (f; after Maniar & Piccoli, 1989). Data for the Raohe pillow basalts are cited from Zhou et al., 2014 and Zeng et al., 2018.

Considering the chondrite-normalized REE patterns, the Tongjiang-Fuyuan basalts have relatively higher ΣREE (86.27–95.27 ppm) and LREE/HREE (4.55–5.21 ppm), and show strong LREE enrichment, with (La/Yb) _N ratios ranging from 6.95 to 7.15. All the facts suggest the basalts have the geochemical characteristics of ocean island basalt (OIB) affinity (Fig. 7a). Similarly, the Tongjiang-Fuyuan gabbros (17TJ5-1A, B) also show the characteristics of the OIB-affinity (Fig. 7a).

Figure 7. Chondrite normalized REE patterns (a and c, normalization values after Boynton, 1984) and primitive mantle normalized trace elements spider diagram (b and d, normalization values after Sun and McDonough, 1989) of Tongjiang-Fuyuan area, NE China. (a, b) basalts and gabbros. (c, d) granodiorites. Data for Raohe pillow basalts are cited from Zhou et al., 2014 and Zeng et al., 2018. OIB = ocean island basalt. E-MORB = Enriched mid-ocean ridge basalt. N-MORB = Normal mid-ocean ridge basalt.

In the primitive mantle (PM) normalized multi-element diagram (Fig. 7b), both the Tongjiang-Fuyuan gabbros and basalts show similar patterns to the present-day OIB-affinity, which are geochemically enriched in high field strength elements (HFSE; such as Nb, Ta, Hf and Zr) and relatively enriched in large-ion lithophile elements (LILE; e.g. Rb, Sr and Ba), together with the slightly negative Eu (Eu/Eu* = 0.87–0.91) anomalies (Fig. 7b).

4.b.2 Major and Trace elements in intermediate-acidic rocks (granodiorites)

Total 8 granodiorites were selected from Tongjiang-Fuyuan for whole-rock major and trace element analysis (Table S1).

The granodiorites have SiO₂ contents ranging from 65.18 to 71.06 wt%, Na₂O from 2.31 to 4.39 wt%, and K₂O from 2.26 to 3.98 wt%. Additionally, all the granodiorites are characterized by relatively low TiO₂ (0.45–0.60 wt%), Al₂O₃(14.26–15.87 wt%), Fe₂O₃ ^T (3.19–4.28 wt%), MgO (1.02–2.31 wt%) in their geochemical compositions. In the A/NK vs. A/CNK diagram (Fig. 6d), they show quasi-aluminous to weakly peraluminous characteristics. In the Na₂O+K₂O-SiO₂ diagram (Fig. 6e), all the granodiorites fall in the granodiorite category. In the K₂O-SiO₂ diagram (Fig. 6f), the granodiorites show a transitional character of high K-medium K-Ca alkaline series.

Granodiorites have relatively high ΣREE (124.29–193.57 ppm) and show strongly enriched in LREE and extremely depleted in HREE (LREE/HREE = 5.51–12.03), with (La/Yb) _N ratios ranging from 5.04 to18.38 (Fig. 7c). In the primitive mantle (PM) normalized multi-element diagram (Fig. 7d), all granodiorites show enrichment of LILEs (e.g. Rb, Ba and Sr) and depletion of HFSEs (such as Nb, Ta, Zr and Hf) and P, with negative Eu (Eu/Eu*=0.45–0.71) anomaly (Fig. 7d).

5 Discussion

5.a Origin of the Tongjiang-Fuyuan Mesozoic magmatic rocks

5.a.1 Mafic rocks

The active elements of Rb, K, Ba and Sr are greatly affected by weathering in the process of metamorphism and alteration. On the contrary, the trace elements, e.g. Nb, Ta, Zr, Hf and Y, are inactive elements, and their contents will not change due to weathering, alteration and certain metasomatism. Moreover, its content does not vary with the degree of partial melting of mantle rocks and the degree of separation and crystallization of basaltic magma (Li, 1992; Janney and Castillo, 1996).

According to the analysis, the mafic rocks (basalts and gabbros) have high TiO₂, MgO and low Al₂O₃, CaO, P₂O₅ and K₂O. Furthermore, these samples exhibit an enrichment in LILEs (Rb, Sr and Ba), and they display right-inclined trend curves on both the trace element spider and rare earth element distribution diagrams, with ΣLREE/ΣHREE ratios ranging from 4.55 to 5.21. Additionally, there is no or slightly negative δEu anomalies (0.87–0.91). All of the analysed mafic rocks have relatively high Nb and Ta contents (Nb > 1.73 ppm, Ta > 0.14 ppm), distinguishing them from arc-related basalts. In the Ti/100-Zr-3*Y (Fig. 8a), Nb*2-Zr/4-Y (Fig. 8b) and Zr/Yb-Zr (Fig. 8c) diagrams, the Tongjiang-Fuyuan basalts and gabbros plot in the within-plate field, consistent with that of the previous reported Raohe pillow basalts. In addition, in the Th/Yb-Nb/Yb diagram (Fig. 8d), the Tongjiang-Fuyuan mafic rocks plot in the MORB-OIB array. Similarly, in the Nb/La-La/Yb (Fig. 8e) and Th/Nb-La/Yb (Fig. 8f) diagrams, they also plot in oceanic islands. In general, Tongjiang-Fuyuan mafic rocks (gabbros and basalts) are mainly formed in intraplate environments and the oceanic island-related tectonic setting. Therefore, it can be concluded that the Tongjiang-Fuyuan mafic rocks are fragments of oceanic island seamount (Fan et al., 2021).

Figure 8. Discriminant diagrams of the Tongjiang gabbros and basalts, Ti/100-Zr-3*Y, 2Nb-Zr/4-Y(a,b; after Pearce & Cann, 1973), Zr/Yb-Zr, Th/Yb-Nb/Yb(c,d; after Pearce & Norry, 1979), Nb/La-La/Yb and Th/Nb-La/Yb(e,f; after Hollocher et al., 2012). Data for Raohe pillow basalts are cited from Zhou et al., 2014 and Zeng et al., 2018. In the Ti/100-Zr-3*Y diagram, A-Island-arc tholeiite; B-Mid-Ocean ridge basalt/ Calc-alkali basalt/Island-arc tholeiite; C-Calc-alkali basalt; D-Within-plate basalt. In the 2Nb-Zr/4-Y diagram, AI-Within-plate alkali basalt; AII-Within-plate tholeiite; B-Plume-influenced mid-ocean ridge basalt; C-Within-plate tholeiite/ volcanic arc basalt; D-Volcanic arc basalt/ Normal mid-ocean ridge basalt. In the Zr/Yb-Zr diagram, A-Island-Arc Basalts; B-Mid-Ocean Ridge Basalt; C-Within-Plate Basalts; D-Mid-Ocean Ridge Basalt/ Island-Arc Basalt. OIB = Ocean Island basalt. E-MORB = Enriched mid-ocean ridge basalt. N-MORB = Normal mid-ocean ridge basalt.

Compared with the Raohe Complex, the previous studies suggested that they were a set of mafic-ultramafic rocks composed of peridotite, pyroxene peridotite, hornblendite, cumulate gabbro, diabase and pillow basalt (Tian et al., 2006; Bi et al., 2016, 2017a; He et al., 2016, 2017b; Cao et al., 2019; Han et al., 2022). Regarding the tectonic setting of these mafic-ultramafic rocks, Zhang and Zhou (2001) suggested that the basalts or pillow lavas developed in the Raohe area were formed in an oceanic environment and have the characteristics of OIB-affinity. Further, Zhou et al. (2014) regarded that the geochemical characteristics of pillow basalts exposed in the Raohe-Guanmen area are similar to those of OIB. As discussed before, the gabbros and basalts in the Tongjiang-Fuyuan area are also originated in intraplate environments and oceanic island-related tectonic settings, exhibiting the characteristics of OIB, which is consistent with characteristics of the Raohe ophiolitic basalts.

As is well known, the OIB-related magma is formed through contributions from different endmember components such as mantle plume, asthenosphere, lithospheric mantle (e.g. Xu, 2002; Zeng et al., 2018). Besides, the OIB samples show much higher ratios of Ta/Hf, Th/Yb and Ta/Yb, and such strong enrichment in highly incompatible elements may indicate partial melting of a mantle source (Aldanmaz, 2002). Further, Mg# values (64–75, avg. = 67) of the mafic rocks in this study are lower than the primary magma range (68–75), indicating that the magma undergoes crystallization fractionation. The Tongjiang gabbros and basalts are high in MgO contents (3.68–19.72 wt%, 8.95 wt% on average), enriched in LREEs, and have ‘Th’ peaks on the normalized spider diagram of trace elements (Fig. 7b), implying that the source area may have the characteristics of a mantle plume (Hou et al., 1996). A similar conclusion has been drawn for the OIB magmatic rocks in the Raohe area (e.g. Wang et al., 2013).

In summary, the Tongjiang-Fuyuan gabbros and basalts in this study show obvious characteristics of OIB and generally relate to the mantle plume within the Palaeo-Pacific plate, consistent with the origin of the Raohe ophiolites (Zhou et al., 2014).

5.a.2 Intermediate-acidic intrusive rocks

The granodiorites, collected from the Tongjiang-Fuyuan area, have relatively high SiO₂, Al₂O₃ and Na₂O+K₂O contents, and low MgO, Fe₂O₃ and CaO, indicating that the magma originated from partial melting of crustal materials (Barbarin, 1999). In addition, these samples are enriched in LREEs and LILEs, and deficient in HREEs and HFSEs, indicating that the magma was formed in the lower crust (Taylor & McLennan, 1985; Hofmann, 1988; Wu et al., 2007; Zhang et al., 2008).

The I-type granites were derived from the lower part of the continental crust at the margin of convergent plates, and the source rocks were probably mantle-derived underplating (Pitcher, 1993). In the FeO^T/MgO-(Zr+Nb+Y+Ce) diagram (Fig. 9a) and Ce-SiO₂ diagram (Fig. 9b), the granodiorites plot in the I-type granite field. In the Sr/Y-Y diagram (Fig. 9c) and (La/Yb) _N-Yb_N diagram (Fig. 9d), the granodiorites plot in the island-arc field. Further, in the Rb-Y+Nb diagram (Fig. 9e) and Nb-Y diagram (Fig. 9f), the granodiorites fall within the volcanic island-arc area. Therefore, we propose that the granodiorites from the Tongjiang-Fuyuan area are I-type granites, which may be formed in the lower crust and polluted by mantle-derived materials, and generally formed in a volcanic arc environment.

Figure 9. Discrimination diagrams of Tongjiang-Fuyuan granodiorites, FeO^T/MgO-(Zr+Nb+Y+Ce) (a; after Whalen et al., 1987), Ce-SiO₂ (b; after Whalen et al., 1987), Sr/Y-Y (c), (La/Yb) _N-Yb _N (d; after Hansen et al, 2002), Rb-Yb+Nb (e), Nb-Y (f; after Pearce et al, 1984). Syn-COLG = syn-collisional-granites; VAG = volcanic arc granites; ORG = oceanic ridge granites; WPG = within-plate granites.

Besides, the REE distribution mode of the granodiorites in this area has a right-inclined trend, and are enriched in LILEs, and deficient in HFSEs, especially Ti, Nb and Ta. Trace element characteristics suggest that the granodiorite in the Tongjiang-Fuyuan area might originate from partial melting of the lower crust (Yu et al., 2017). In conclusion, all the facts suggest that the granodiorites from Tongjiang-Fuyuan are island-arc magmatic rocks, and the magma may originate from the partial melting of the lower crust.

5.b Formation age of the Tongjiang-Fuyuan Mesozoic magmatic rocks

Field investigation and rock associations show that the predominant rock types in the Tongjiang-Fuyuan region are siliceous rocks, gabbro, metamorphic sandstone, basalt and so on, which are typical ophiolite mélange types and are present with granodiorite intrusion. The zircon LA-ICP-MS U-Pb data from a gabbroic sample (17TJ5-1) and three granodiorite samples (17TJ3, 17FY3 and 17FY9) yield the weighted mean age of 95 ± 2 Ma (Fig. 5b) and 93–95 Ma (Fig. 5a, c and d).

According to the previous studies, the Raohe Complex is considered as the subduction-accretionary product of the Palaeo-Pacific plate (Zhou et al., 2014; Bi et al., 2016, 2017a, 2017b; Ge et al., 2016; Zhou et al., 2017; Han et al., 2022), and their formation ages of magmatic rocks (pillow basalt and gabbro) and intermediate acid intrusive rocks (granite) in the Raohe area are 167–168 Ma and 128–129 Ma, respectively (Cheng et al., 2006; Zhou et al., 2014; Zeng et al., 2019; Han et al., 2022). In this study, zircon age analysis shows that gabbro and granodiorite in the Tongjiang-Fuyuan area are 93–95 Ma. Therefore, the Tongjiang-Fuyuan mafic rocks and intermediate acid intrusive rocks resemble the Raohe Complex in nature but are younger in age, proving that they are the results of continuous subduction-accretion of the Palaeo-Pacific plate.

In addition, in the northeastern NE China, synchronous magmatism was also discovered by predecessors, for instance, the gabbros (96–101 Ma) in the eastern Jixi Basin (Zhu et al., 2009), the diorites (97.5 Ma) in the Shuangyashan Basin (Zhang et al., 2009), the olive trachyandesites (88 Ma) in the Songliao Basin (Wang et al., 2009), the dacites (93.2 Ma) in the Suifenhe region (Ji et al., 2007), the alkaline basalts (81.6 Ma) in Qujiatun of Liaodong (Wang et al., 2006) and so on. All of the Late Cretaceous magmatic rocks were interpreted as the subduction-related magmatism, significantly suggesting the continuous subduction of the Palaeo-Pacific plate beneath the eastern margin of the JB till to Late Cretaceous (∼80 Ma).

5.c Tectonic implications

5.c.1 Subduction and rollback of the Palaeo-Pacific plate

During the Mesozoic, the tectonic evolution of the Northeast Asian continental margin was mainly influenced by the subduction of the Palaeo-Pacific plate, which has been recognized by many geologists. A large number of magmatic belts were formed along the Northeast Asian continental margin along with the subduction of the Palaeo-Pacific plate (e.g. Wu et al., 2007, 2011; Sun et al., 2013, Xu et al., 2013; 2015a; Wilde, 2015). Notably, a Mesozoic subduction-related magmatic belt with NNE-trending distribution is widely developed in the eastern continental margin of the Northeast Asia, extending from the Russian Far East, via NE China to the SW Japan (e.g. Isozaki, 1997; Lin et al., 1998; Shao & Tang, 2015; Li et al., 2020). In the NE China, these magmatic rocks are mainly distributed in the SXB (Such as Yichun area, Nenjiang area, and Tuanshanzi area; Zhang et al,.2007; Ji et al., 2019; Pei et al., 2008), in the JB (such as Boli area, Jixi area and Jiamusi area; Sun et al., 2013, 2014) and in the NT (such as Raohe area, Yuejinshan area, Tongjiang-Fuyuan area; Yu et al., 2013; Zhou et al., 2014, 2015). A large number of geochronological and geochemical studies have been carried out on the magmatic rocks from the SXB, JB and NT and proposed that the formation age of various magmatic rocks is mainly Mesozoic, and their tectonic environment is closely related to the westward subduction of the Palaeo-Pacific plate (e.g. Cheng et al., 2006; Liu et al., 2008; Li et al., 2011; Wu et al., 2011; Zhang et al., 2011; Yu et al., 2013; Sun et al, 2013, 2015; Zhou et al., 2014, 2015; Bi et al, 2015; Wang et al., 2015, 2016; Yang et al, 2015; Zhu et al, 2015; Ge et al, 2016, 2017, 2019; Liu et al, 2017a, 2017b; Zeng et al., 2017, 2018; Dong et al, 2019; Ji et al, 2019; Li et al., 2020).

Meanwhile, previous studies show that in the late Early Cretaceous, the magmatic rocks in northeast China are mainly distributed in EB, XB, SXB, JB and NT. (e.g. Zhou et al., 2014, 2015; Bi et al, 2015; Wang et al., 2015, 2016; Yang et al, 2015; Zhu et al, 2015; Zeng et al, 2017, 2018; Liu et al., 2019; Li et al., 2020). The distribution characteristics of magmatic rocks during the Late Early Cretaceous period suggest that the Palaeo-Pacific plate began a wide range of low-angle subduction to Eurasia, and the magmatic range in NE China contracted to the east, suggesting the eastward drift of the Eurasia continent and the rollback of the Palaeo-Pacific subduction plate (e.g. Engebretson et al., 1985; Maruyama et al., 1997; Yu et al., 2013; Sun et al., 2013, 2014; Li et al., 2020).

This research gathers various ages from the SXB to the NT based on other investigations on the chronology of magmatic activity in NE China (Fig. 10). The ages of the Mesozoic magmatic rocks in the SXB, JB, and NT areas reveal a west-to-east and old-to-young trend of magmatic activity in NE China, especially during the Cretaceous (145–190 Ma) (Fig. 10). Such a pattern, which indicates the tectonic rollback process of the Palaeo-Pacific plate, is perfectly consistent with the subduction rollback pattern revealed by previous researchers (Sun et al., 2013, 2014; Li et al., 2020).

Figure 10. Age distributions of the Mesozoic subduction-related magmatic rocks in the NT (a), JB (b) and SXB (c).

Data source: Bi et al., 2015; Ji et al., 2019; Pei et al., 2008; Sun et al., 2013, Wu et al., 2011; Yu et al., 2013; 2014; Zhang et al., 2007, 2011; Zhou et al., 2014, 2015.

5.c.2 Mesozoic tectonic evolution of the eastern NE China

The formation and evolution of the East Asian continental margin have involved complicated tectonomagmatic processes, including subduction, accretion and magmatism. Furthermore, the tectonic processes are characterized by the ‘transform’ nature or strike-slip displacement of individual terranes or blocks of different tectonic history (e.g. Kojima, 1989; Khanchuk, 2001, 2006; Kirilova, 2003; Kemkin, 2008; Isozaki et al., 2010; Abrajevitch et al., 2012; Maruyama et al., 1997; Zonenshain et al., 1990a; 2004b; Tazawa, 2004). The tectonic evolution and crustal formation in NE China are intimately linked to the interaction between the Palaeo-Pacific (so-called Izanagi) and Eurasian plates. Besides, the quasi-continuous magmatism in Late Cretaceous of the NE China (Fig. 10) indicates that the Palaeo-Pacific plate subduction was long-term active. An important implication from the present geochronological work is that during Late Cretaceous the Palaeo-Pacific plate motion probably changed from a parallel or sub-parallel (magmatic quiescence) to oblique (active arc magmatism) relative to the continental margin of Sikhote-Alin (Zhou et al., 2014; Tang et al., 2018; Han et al., 2022). Late Cretaceous rapid sea-floor spreading at about 100 Ma (Larson & Pitman, 1972; Larson, 1991) induced highly active subduction and led to voluminous magmatism in the entire circum-Pacific areas (Jahn, 1974).

A large part of the Russian Far East is built up of accretionary complexes that formed along the convergent margin during the Cretaceous. A complex distribution of coeval complexes as well as juxtaposition of different age units indicates a considerable margin-parallel translation of terranes, which was represented by the sinistral strike-slip fault systems in the region, of which the Central Sikhote-Alin Fault is the most celebrated (e.g. Khanchuk, 2001, 2006; Otsuki, 1992; Sengor & Natal’in, 1996; Tazawa, 2004). A palaeomagnetic study led Abrajevitch et al. (2012) to hypothesize that the West Sakhalin Basin has moved from sub-equatorial latitude during the early Cretaceous to about 40°N by the late Cretaceous. Similarly, the NT of NE China was probably accreted to the Asian continental margin from low latitude during the late Mesozoic (Mizutani & Kojima, 1992).

As described earlier, the NT is mainly divided into Yuejinshan Complex and Raohe Complex (Bi et al., 2015, 2016, 2017a, 2017b; Sun et al., 2015a; Wilde, 2015; Zeng et al., 2019; Li et al., 2020; Xu et al., 2020; Han et al., 2022). Zircon chronology and geochemistry indicated that the Yuejinshan Complex mainly consisted of siliceous rocks, MORB and broken seamount fragments (Zhou et al., 2014, 2018; Zhang et al., 2020). The formation age of the complex is 210–180 Ma (Zhou et al., 2014; Zeng et al., 2018), while their protolith has a formation age of 310–270 Ma (Zhou & Li, 2017; Li et al., 2020). Besides, the Yuejinshan Complex may have formed because of the westward subduction of the Palaeo-Pacific plate (Zhou et al., 2014; Li et al., 2020; Han et al., 2022). The Raohe complex consists primarily of OIB, limestone, siliceous rock and seamount fragments (Zeng et al., 2019; Li et al., 2020). It is emplaced between 133 Ma and 126 Ma (Zeng et al., 2017, 2019), and its protoliths range in age is from 228 Ma to 116 Ma (Cheng et al., 2006; Wang et al., 2016; Li et al., 2020). It is believed that the Raohe Complex is the product of the subduction of the Palaeo-Pacific plate (Zhou et al., 2014; Zeng et al., 2019; Li et al., 2020; Han et al., 2022). However, the Late Cretaceous rocks in NT have been scarcely studied, and their link with the Palaeo-Pacific plate subduction is poorly understood.

In this paper, we collect mafic rocks (gabbro, basalt) and intermediate-acidic intrusive rocks (granodiorite) through field investigation in the northern of the NT (Tongjiang-Fuyuan area). Zircon LA-ICP-MS U-Pb results show that the formation age of the Tongjiang-Fuyuan mafic rocks (gabbros) and intermediate-acidic intrusive rocks (granodiorites) is 93–95 Ma. Besides, the results of geochemistry show that mafic rocks (gabbros) and intermediate-acidic intrusive rocks (granodiorites) in the Tongjiang-Fuyuan area are similar to Raohe Complex in nature and younger in age; therefore, it is newly named Tongjiang-Fuyuan Complex in this paper. More significantly, all the Late Cretaceous magmatic rocks show the characteristics of the subduction-related magmatism, we thus interpreted that the Tongjiang-Fuyuan Complex is also the product of the continuous subduction of the Palaeo-Pacific plate.

Based on the above evidence and the combination of the previous studies, a tectonic model of subduction accretionary of the Late Triassic to Late Cretaceous Palaeo-Pacific plate in the NT is proposed, which can be divided into three stages. (1) Late Triassic to Early Jurassic (210–180 Ma) is the time when the onset of westward-directed accretion related to Pacific-Plate plate subduction. The Palaeo-Pacific plate subducted westward and formed the emplacement of the Yuejinshan Complex on the eastern margin of the JB (e.g. Zhou et al., 2014, 2015; Zeng et al., 2018; Li et al., 2020) (Fig. 11a). (2) During the Early Jurassic to the Early Cretaceous (180–130 Ma), the Palaeo-Pacific plate with Early Jurassic seamounts collided with the East Asian continental margin and brought associated limestone, bedded chert and siliceous shale, and a large number of terrigenous debris and seamount fragments were tectonically mixed and accumulated in the trench to form the Raohe Complex (e.g. Zhou et al., 2014; Li et al., 2020; Han et al., 2022) (Fig. 11b). (3) During the Late Cretaceous (90–100 Ma), the Palaeo-Pacific plate continued to subduct, and with the change of subduction angle, the Palaeo-Pacific plate began to roll back (Yu et al., 2013; Sun et al., 2014; Li et al., 2020; Han et al., 2022). Along with the accumulation of seamount fragments, siliceous rocks and OIB as well as the invasion of the granitic rocks, the Tongjiang-Fuyuan Complex was formed (Fig. 11c).

Figure 11. Map of the subduction-accretionary pattern of the Late Triassic-Late Cretaceous Palaeo-Pacific plate (NT as an example).

In short, these successive accretionary complexes, gradually younger to the east in the NE China, significantly indicate the subduction and rollback process of the Palaeo-Pacific plate (Zhang et al., 2011; Sun et al., 2013).

6. Conclusion

Based on the zircon LA-ICP-MS U-Pb ages and geochemical data presented above, we draw the following conclusions.

1. 1. Field observation shows that the rock association in the Tongjiang-Fuyuan area resembled the ophiolite suite and was newly defined as the Tongjiang-Fuyuan Complex. Zircon U-Pb ages show the formation age of the Tongjiang-Fuyuan mafic rocks (gabbros), and intermediate-acidic intrusive rocks (granodiorites) are 93–95 Ma, that are slightly younger than the similar rock associations developed in the Raohe Complex.

2. 2. As Mesozoic magmatic rocks in the Tongjiang-Fuyuan complex, mafic rocks (gabbros and basalts) show geochemical features similar to those of OIB, and their primitive magmas are generated from the mantle thermal plume. Meanwhile, intermediate-acidic intrusive rocks (granodiorites) are I-type granites, forming in a magmatic arc setting, and probably originated from the partial melting of the lower crust.

3. 3. Combined with the coeval igneous rock associations and regional tectonic evolution, we conclude that the Late Cretaceous magmatic rocks in the Tongjiang-Fuyuan area are the result of continuous subduction of the Palaeo-Pacific plate beneath the eastern margin of the JB. Furthermore, the distribution of magmatic rocks, which are gradually younger to the east, significantly reflects the rollback of the subducted Palaeo-Pacific plate.