3.a. Zircon U–Pb dating

Representative zircon grains were separated from the DBZG porphyritic dolerite by standard density and magnetic separation techniques at Langfang Chengxin Geological Service, Hebei Province, China. Their external and internal structures were observed microscopically under transmitted and reflected light, and by cathodoluminescence (CL) methods at the Micro-area Laboratory, Institute of Geology, Chinese Academy of Geological Sciences (IGCAGS), Beijing, China.

U–Pb dating of zircon separates was performed by laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) at the Ministry of Natural Resources Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), Beijing, China, following the techniques described by Wu et al. (Reference Wu, Yang, Xie, Yang and Xu2006) and Hou et al. (Reference Hou, Li, Zou, Qu, Shi and Xie2007), with an ablation spot of ~32 μm in diameter.

3.b. Major- and trace-element analyses

Whole-rock major- and trace-element compositions were determined at the China National Research Center of Geoanalysis, CAGS. Sample preparation involved the trimming of altered surfaces, washing with deionized water, and crushing and powdering in an agate mill. Major elements were analysed by X-ray fluorescence (XRF) spectrometry. Analytical accuracy is estimated to be ±1% for SiO₂ and ±2% for other oxides. Trace elements including REE were determined by ICP-MS. Analytical uncertainties and detailed analytical procedures are described by Qi et al. (Reference Qi, Hu and Gregoire2000). Analytical precision for trace elements was better than ±5%.

3.c. Rb–Sr and Sm–Nd isotopic analyses

Rb–Sr and Sm–Nd isotopic analyses were performed at the Centre of Modern Analysis, Nanjing University, Nanjing, China, using a VG 354 MS. Sample powders were spiked with mixed isotopic tracers before dissolution in HF–HNO₃–HClO₄. Analysis of NBS987 and La Jolla standards over the past 5 years has yielded mean ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.710342 ± 0.000040 and 0.511840 ± 0.000008, respectively. Procedural blanks were <1 ng for Sr and <60 pg for Nd (Wang et al. Reference Wang, Yang, Chen, Zhang and Rao2007).

3.d. Zircon Hf isotopic analyses

Zircon Hf analyses of the grains used for U–Pb dating were performed by LA-ICP-MS (Neptune Plus and Compex pro 193nm LA system) at the Continental Tectonics and Dynamics Laboratory, IGCAGS. Ablation-spot diameter was 44 or 32 μm, depending on the zircon size. Zircon standard GJ-1 was used as a reference material. Detailed procedures are described by Hou et al. (Reference Hou, Li, Zou, Qu, Shi and Xie2007). The weighted-mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of GJ-1 was 0.282015 ± 0.000008 (2σ, n = 10), consistent with reported values (Hou et al. Reference Hou, Li, Zou, Qu, Shi and Xie2007). Data acquisition protocols are described by Wu et al. (Reference Wu, Yang, Xie, Yang and Xu2006). The following parameters were used in calculations of ɛ _Hf(t) and Hf model ages: (¹⁷⁶Lu/¹⁷⁷Hf)_DM = 0.0384, (¹⁷⁶Hf/¹⁷⁷Hf)_DM = 0.28325 (Griffin et al. Reference Griffin, Pearson, Belousova, Jackson, van Achterbergh, O’Reilly and Shee2000); (¹⁷⁶Lu/¹⁷⁷Hf)_CHUR = 0.0332, (¹⁷⁶Hf/¹⁷⁷Hf)_CHUR,0 = 0.282772 (Blichert-Toft et al. Reference Blichert-Toft, Chauvel and Albarède1997); (¹⁷⁶Lu/¹⁷⁷Hf)_CC = 0.015 (Griffin et al. Reference Griffin, Wang, Jackson, Pearson, O’Reilly, Xu and Zhou2002); and λ_Lu = 1.867 × 10^–11 a^–1 (Söderlund et al. Reference Söderlund, Patchett, Vervoort and Isachsen2004).

4.a. U–Pb zircon geochronology

Zircon grains separated from porphyritic dolerite sample DBZG-1 are subhedral to euhedral, prismatic, contain clear oscillatory zoning, and have Th/U ratios of 0.29–1.31, indicative of a magmatic origin. They yield a weighted-mean ²⁰⁶Pb/²³⁸U age of 187.6 ± 1.5 Ma (n = 22; MSWD = 0.65; Table 1; Fig. 3), which is interpreted as the crystallization age of the DBZG porphyritic dolerite pluton.

Fig. 3.

Representative U–Pb concordia diagram for zircon grains from mafic sample DBZG-01. Inset shows the locations of ablation spots.

Table 1.

LA-ICP-MS U–Pb isotopic data for zircons from the Dabazigou mafic dykes

4.b. Major- and trace-element geochemistry

Whole-rock geochemical compositions of the analysed dolerite samples are given in Tables 2 and 3. Dolerite rocks have relatively low and uniform SiO₂ (46.18–48.39 wt %), Na₂O (2.99–4.77 wt %) and K₂O (0.41–1.33 wt %) contents, with MgO contents of 7.15–9.31 wt % and Mg numbers of 60–64. All samples plot in the alkaline-basalt field in total-alkalis v. SiO₂ (TAS), and Zr/TiO₂ v. Nb/Y diagrams (Fig. 4), unlike the Lower Jurassic calc-alkaline basalts of the Zhangguangcai Range.

Fig. 4.

Geochemical classification of the DBZG mafic rocks. (a) SiO₂ v. (Na₂O + K₂O) and (b) Nb/Y v. Zr/TiO₂ (×10^–4) (Winchester & Floyd, Reference Winchester and Floyd1977).

Table 2.

Major (%) element data for the Dabazigou mafic dykes

Note: LOI: Loss on ignition; Mg# = 100 × Mg² ± (Mg²⁺ + Fe²⁺).

Table 3.

Trace element (ppm) data for the Dabazigou mafic dykes

The dolerites are characterized by smooth chondrite-normalized REE patterns, displaying moderate LREE enrichment similar to OIB, with a narrow range of Eu/Eu* (0.70–1.03) and (La/Yb)_N (5.67–6.52) values (Table 3; Fig. 5a). The primitive-mantle-normalized trace-element diagram (Fig. 5b) indicates weak Nb–Ta enrichment and no obvious Zr, Hf or Ti depletion, similar to but lower values than average OIB. However, these obtained values are significantly higher than those of subduction-zone basalts. The analysed samples have Nb/U ratios of 31–37, similar to those of average OIB (47 ± 10; Hofmann et al. Reference Hofmann, Jochum and Seufert1986).

Fig. 5.

(a) Chondrite-normalized REE patterns, and (b) primitive-mantle-normalized trace-element spider diagrams for the DBZG diabase porphyrite. Normalizing values for chondrite and primitive mantle, and OIB, N-MORB and E-MORB values are from Sun & McDonough (Reference Sun, McDonough, Saunders and Norry1989). Other data sources are: Middle Okinawa Trough (Shinjo et al. Reference Shinjo, Chung, Kato and Kimura1999); Andean back-arc basalt (Hernando et al. Reference Hernando, Aragón, Frei, González and Spakman2014); subduction-zone basalt, with the lower and upper limits defined by ‘average’ low-K and high-K basalts, respectively (Tatsumi & Eggins, Reference Tatsumi and Eggins1995; Xia, Reference Xia2014).

4.c. Rb–Sr and Sm–Nd isotopic compositions

DBZG porphyritic dolerite samples have (⁸⁷Sr/⁸⁶Sr)_i values of 0.7033–0.7044 and relatively uniform positive ɛ _Nd(t) values (2.3–3.2; Table 4; Fig. 6a), suggesting that their magmas had a depleted-mantle source. They have yielded a corresponding single-stage Nd model age (T_DM) of 0.75–0.92 Ga.

Fig. 6.

(a) Sr–Nd isotopic diagram, and (b) ɛ _Hf(t) v. U–Pb zircon age diagram for the Dabaizigou mafic intrusion in NE China. Data sources: MORB, OIB and mantle array (Zhang et al. Reference Zhang, Sun, Zhou and Ying2005); I-type granitoids (Li et al. Reference Li, Guo, Li, Li and Zhao2014; Guo et al. Reference Guo, Li, Fan, Li, Zhao, Huang and Xu2015); Mashan Group (Wu et al. Reference Wu, Jahn, Wilde and Sun2000); Early Jurassic mafic intrusive rocks in the Yanji area (Guo et al. Reference Guo, Li, Fan, Li, Zhao, Huang and Xu2015); Permian mafic intrusive rocks in the Yanji area (Guo et al. Reference Guo, Li, Fan, Li, Zhao and Huang2016); Early Permian olivine–gabbro of the Zhuangguangcai Range (Feng et al. Reference Feng, Liu, Niu and Yang2018); Early Jurassic mafic rocks of the Lesser Xing’an Range (Yu et al. Reference Yu, Wang, Xu, Gao and Pei2012). CAOB, Central Asian Orogenic Belt; YFTB, Yanshan Fold-and-Thrust Belt (Yang et al. Reference Yang, Wu, Shao, Wilde, Xie and Liu2006).

Table 4.

Sr–Nd isotopic ratios for the representative Dabazigou mafic dykes and Zhongxiantun granites

Chondrite Uniform Reservoir (CHUR) values (⁸⁷Rb/⁸⁶Sr = 0.0847, ⁸⁷Sr/⁸⁶Sr = 0.7045, ¹⁴⁷Sm/¹⁴⁴Nd = 0.1967, ¹⁴³Nd/¹⁴⁴Nd = 0.512638) are used for the calculation. λ _Rb = 1.42 × 10⁻¹¹ a⁻¹ (Steiger & Jäger, Reference Steiger and Jäger1977); λ _Sm = 6.54 × 10⁻¹² a⁻¹ (Lugmair & Harti, Reference Lugmair and Harti1978).

4.d. Zircon Hf isotopic composition

The DBZG porphyritic dolerite samples have ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.001008–0.011259, (¹⁷⁶Hf/¹⁷⁷Hf)_i ratios of 0.282895–0.283063 and ɛ_Hf(t) values of 8.5–17.1 (Table 5; Fig. 6b). Their single-stage Hf model ages (T_DM1) range between 502 Ma and 269 Ma (discarding one analysis, spot 9, of 149 Ma).

Table 5.

Zircon Hf isotope date for the Dabazigou mafic dykes and Zhongxiantun granites

ɛ _Hf(t) = 10 000{[(¹⁷⁶Hf/¹⁷⁷Hf)_S – (¹⁷⁶Lu/¹⁷⁷Hf)_S × (e ^λt – 1)]/[(¹⁷⁶Hf/¹⁷⁷Hf)_CHUR,0 – (¹⁷⁶Lu/¹⁷⁷Hf)_CHUR × (e ^λt – 1)] –1}

T _DM1 = 1/λ × ln{1 + (¹⁷⁶Hf/¹⁷⁷Hf)_S – (¹⁷⁶Hf/¹⁷⁷Hf)_DM]/[(¹⁷⁶Lu/¹⁷⁷Hf)_S – (¹⁷⁶Lu/¹⁷⁷Hf)_DM]}

ƒ_Lu/Hf = (¹⁷⁶Lu/¹⁷⁷Hf)_S/(¹⁷⁶Lu/¹⁷⁷Hf)_CHUR-1

The ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of chondrite and depleted mantle at the present are 0.282772 and 0.0332, 0.28325 and 0.0384, respectively (Blichert-Toft & Albarède, Reference Blichert-Toft and Albarède1997; Griffin et al. Reference Griffin, Pearson, Belousova, Jackson, van Achterbergh, O’Reilly and Shee2000). λ = 1.867 × 10–11 a^–1 (Söderlund et al. Reference Söderlund, Patchett, Vervoort and Isachsen2004). (¹⁷⁶Lu/¹⁷⁷Hf)_C = 0.015, t = crystallization age of zircon.

5.a. Mantle melt source

The Lower Jurassic DBZG mafic rocks have relatively uniform Nd and Hf isotopic compositions with ɛ _Nd(t) and ɛ _Hf(t) values of 2.3–3.2 and 8.5–17.1, respectively. On a Sr–Nd isotopic diagram and a ɛ _Hf(t) v. U–Pb zircon age diagram, all the analysed rocks plot above the mantle array (Vervoort et al. Reference Vervoort, Patchett, Blichert-Toft and Albarède1999) and east-CAOB fields (Fig. 6a, b), and slightly above the Lower Permian olivine–gabbro from the Zhangguangcai Range and the Lower Jurassic mafic rocks of the Lesser Xing’an Range (Yu et al. Reference Yu, Wang, Xu, Gao and Pei2012; Feng et al. Reference Feng, Liu, Niu and Yang2018). These geochemical features indicate that magmas of the Lower Jurassic DBZG dolerite pluton originated from partial melting of an isotopically depleted mantle source. However, in a primitive-mantle-normalized trace-element diagram (Fig. 5b) the HFSE patterns are higher than those of subduction-zone basalts and mafic rocks of the Lesser Xing’an and Zhangguangcai ranges, but similar to those of OIB (Fig. 5b). In a Th/Yb v. Nb/Yb diagram (Fig. 8f), mafic rocks metasomatized by melts and fluids derived from slab dehydration or subducted sediments plot in the oceanic-arc and continental-arc fields. However, all of the DBZG samples plot along the mid-ocean-ridge basalt (MORB)–OIB array, consistent with their magmas having been derived from a geochemically enriched mantle in an intraplate setting, and with the mantle source being largely unaffected by subduction-related fluids and/or melts (Bi et al. Reference Bi, Ge, Yang, Wang, Tian, Liu, Xu and Xing2017). This result is also compatible with the high Zr/Y ratios and tectonic discrimination diagrams (Fig. 8a, b, c, d, e). Furthermore, in a La/Nb v. La/Ba diagram (not shown), the DBZG mafic rocks plot in the OIB field (Hernando et al. Reference Hernando, Aragón, Frei, González and Spakman2014) based on over 900 analyses of OIB from various ocean islands (Fitton et al. Reference Fitton, James and Leeman1991). It is, therefore, concluded that the magma source of the DBZG mafic rocks was significantly different from that of other Early Jurassic mafic intrusions in the Lesser Xing’an Range that exhibit arc-like trace-element patterns. The latter may have been derived from partial melting of a depleted lithospheric mantle that was metasomatized by slab-derived fluids in a back-arc setting. The DBZG mafic rocks have, on the other hand, a geochemical signature more similar to that of alkaline OIB and alkaline continental basalts.

Fig. 8.

Tectonic discrimination diagrams for the DBZG mafic rocks. (a) Ti/100 v. Zr v. 3Y diagram (Pearce & Cann, Reference Pearce and Cann1973); (b) 2Nb v. Zr/4 v. Y diagram (Meschede, Reference Meschede1986); (c) Hf/3 v. Th v. Ta diagram (Wood, Reference Wood1980); (d) Ti/1000 v. V diagram (Pearce, Reference Pearce2014); (e) Zr v. Zr/Y diagram (Pearce & Norry, Reference Pearce and Norry1979); (f) Nb/Yb v. Th/Yb diagram (Dilek & Furnes, Reference Dilek and Furnes2011; Pearce, Reference Pearce2014).

Two main mantle sources have been proposed for alkaline OIB, namely a metasomatized lithospheric mantle (Niu & O’Hara, Reference Niu and O’Hara2003; Pilet et al. Reference Pilet, Hernandez, Sylvester and Poujol2005, Reference Pilet, Baker and Stolper2008, Reference Pilet, Baker, Müntener and Stolper2011) and the asthenosphere (Fitton et al. Reference Fitton, James and Leeman1991; Farmer, Reference Farmer, Holland and Turekian2003). Pilet et al. (Reference Pilet, Baker, Müntener and Stolper2011) proposed that the origin of alkaline lavas may be related to lithospheric metasomatism, and further suggested that amphibole-bearing veins in a lithospheric mantle may be important components of the source of alkaline OIB and continental basalts (Hernando et al. Reference Hernando, Aragón, Frei, González and Spakman2014). Amphibole-rich veins are interpreted as the cumulates produced by low-degree melting of the underlying asthenosphere during its ascent (Pilet et al. Reference Pilet, Baker, Müntener and Stolper2011). In a modelling study, Pilet et al. (Reference Pilet, Baker, Müntener and Stolper2011) reported that the geochemical signatures of such veins include positive Nb anomalies relative to U and LREEs (due to the compatibility of Nb in amphibole), and low HREE contents and high (La/Yb)_N ratios (due to a garnet-bearing asthenospheric source of metasomatizing fluids). However, we note that these signatures are absent among the geochemical treats of the DBZG mafic rocks, just like the post-caldera basalts of the Payún Matrú volcanic field (PMVF) in the Andean back-arc of western Argentina (Hernando et al. Reference Hernando, Aragón, Frei, González and Spakman2014). We infer, therefore, that a metasomatized lithospheric mantle contribution was not significant in the petrogenesis of the DBZG mafic magmas. We posit, however, that an asthenospheric mantle contribution was important.

5.b. Partial melting conditions

REE and HFSE contents and ratios are not significantly affected by fluid influx or mantle depletion because they are moderately incompatible during partial melting of upper mantle peridotites, according to their partition coefficients (Pearce & Peate, Reference Pearce and Peate1995; Johnson, Reference Johnson1998; Münker, Reference Münker2000). Moreover, Y and Yb contents are buffered in primary melts by residual garnet during melting of mantle peridotites (Johnson, Reference Johnson1998). Thus, melts originating from partial melting of upper mantle peridotites in the garnet stability field display high La/Yb and Sm/Yb ratios, and low Yb contents (Liu et al. Reference Liu, Su, Hu, Feng, Gao, Coulson, Wang, Feng, Tao and Xia2010). In contrast, spinel has similar partition coefficients for Yb, Gd and Dy, with only slight fractionation during melting in the spinel stability field (Green, Reference Green2006; Zhang et al. Reference Zhang, Yuan, Long, Sun, Huang, Du and Wang2016). REE and HFSE contents are, therefore, controlled mainly by the degree of partial melting; the mantle compositions are widely used to discriminate garnet–lherzolite sources from spinel–lherzolite sources and to decipher the degree of mantle melting (e.g. Gurenko & Chaussidon, Reference Gurenko and Chaussidon1995; Münker, Reference Münker2000; Green, Reference Green2006; Zhao & Zhou, Reference Zhao and Zhou2009). In the La v. La/Sm and La/Yb v. Yb diagrams (Fig. 9a, b), the DBZG mafic samples plot in a transition zone between spinel–lherzolite and garnet–lherzolite melting curves, indicating the derivation of their magmas from partial melting of a deeper mantle source in the garnet-facies stability zone and continued melting in the shallower spinel-facies zone, as part of a parabolic melt system with ~10% partial melting (Khogenkumar et al. Reference Khogenkumar, Singh, Bikramaditya Singh, Khanna, Singh and Singh2016). Applying the melting column concept (Plank & Langmuir, Reference Plank and Langmuir2012) at the depth of the spinel–garnet transition for the peridotite solidus at 75–80 km (McKenzie & O’Nions, Reference McKenzie and O’Nions1991; Robinson & Wood, Reference Robinson and Wood1998), we know that in an extensional setting upwelling asthenosphere rises to a relatively shallow level of <80 km, whereby it undergoes decompression melting.

Fig. 9.

(a) La v. La/Sm diagram showing melt curves and lines obtained using the non-modal batch melting model (Aldanmaz et al. Reference Aldanmaz, Yaliniz, Guçtekin and Goncuoglu2008). The parameters and common geochemical reservoirs used in the diagram are as described in Aldanmaz et al. (Reference Aldanmaz, Pearce, Thirlwall and Mitchell2000). The enriched mantle (EM) is assumed to have incompatible element concentrations similar to that proposed in Aldanmaz & Köprübaşi (Reference Aldanmaz and Köprübaşi2006). The mantle array is defined using melt-residual compositional trends from depleted MORB mantle (DMM) and primitive mantle (PM; similar in composition to CI chondrite) compositions. Mantle depletion and enrichment trends are defined by melt extraction from the mantle (towards the residues) and melt addition to the mantle (or influx of material from outside the mantle prior to solid-state mixing and homogenization) respectively. The straight-line array represents mixing between melts from enriched (EM) and depleted mantle (DMM) components, marked by curved lines. (b) Yb v. La/Yb diagram for mafic volcanics of the DBZG mafic rocks. Melt curves are point-average, non-modal, fractional melts of garnet and spinel lherzolites (garnet lherzolite: 0.598 ol, 0.211 opx, 0.076 cpx, 0.115 gt that melts in the proportions 0.05 ol, 0.20 opx, 0.30 cpx, 0.45 gt; spinel lherzolite: 0.578 ol, 0.270 opx, 0.119 cpx, 0.033 sp that melts in the proportions 0.10 ol, 0.27 opx, 0.50 cpx, 0.13 sp (Thirlwall et al. Reference Thirlwall, Upton and Jenkins1994; Baker et al. Reference Baker, Menzies, Thirlwall and MacPherson1997)), and the scale melting fraction is from Rotolo et al. (Reference Rotolo, Castorina, Cellura and Pompilio2006). Distribution coefficients were taken from McKenzie & O’Nions (Reference McKenzie and O’Nions1991). The MORB and OIB fields from Baker et al. (Reference Baker, Menzies, Thirlwall and MacPherson1997) and El-Rahman et al. (Reference El-Rahman, Polat, Dilek, Fryer, El-Sharkawy and Sakran2009) are shown for comparison.

6.a. Possible tectonic settings of OIB-type magmatism

Two main tectonic scenarios have been proposed for the genesis of OIB-type mafic rocks: (i) the ascent of a mantle plume (Burke & Wilson, Reference Burke and Wilson1976; Richards et al. Reference Richards, Duncan and Courtillot1989; Griffiths & Campbell, Reference Griffiths and Campbell1991), such as those OIB alkaline basalts observed in the early and late phases of Hawaiian eruptions (Frey et al. Reference Frey, Wise, Garcia, West, Kwon and Kennedy1990; Moore & Clague, Reference Moore and Clague1992; Borgia & Treves, Reference Borgia, Treves, Parson, Murton and Browning1992; Wanless et al. Reference Wanless, Garcia, Rhodes, Weis and Norman2006) and in some ophiolitic melanges (i.e. Tankut et al. Reference Tankut, Dilek and Onen1998; Sarifakioglu et al. Reference Sarifakioglu, Dilek and Sevin2017); and (ii) tectonic extension of continental lithosphere (McKenzie & Bickle, Reference McKenzie and Bickle1988; Xu, Reference Xu2007), including lithospheric extension and thinning of orogenic crust during a post-collision stage or in a back-arc setting (Dilek & Whitney, Reference Dilek and Whitney2000; Kadioglu et al. Reference Kadioglu, Dilek, Foland, Dilek and Pavlides2006). The former mechanism has been invoked to explain the origin of Mesozoic–Cenozoic alkaline basalts in eastern China (Fan & Hooper, Reference Fan and Hooper1991; Tang et al. Reference Tang, Zhang and Ying2007; Zhang et al. Reference Zhang, Zheng and Zhao2009). A typical case of the latter mechanism is the origin of back-arc basin basalts in the Andes (Hernando et al. Reference Hernando, Aragón, Frei, González and Spakman2014). The ascent of a mantle plume would generally result in large-scale and intensive mafic magmatism over a short period of time; consequently, this mechanism is not favoured for the present study area. A post-collision environment would produce intense magmatism of diverse compositions, including basaltic magmatism displaying within-plate basalt geochemical affinity, bimodal magmatism and A2-type felsic magmatism (von Blanckenburg & Davies, Reference von Blanckenburg and Davies1995; Luca, Reference Luca2004; Dilek & Altunkaynak, Reference Dilek and Altunkaynak2007; Dilek et al. Reference Dilek, Imamverdiyev and Altunkaynak2010), similar to that observed in the present study area, where the Maoershan Group bimodal magmatism (Tang et al. Reference Tang, Xu, Wang, Gao and Cao2011) occurred along with A-type granitoids (Wu et al. Reference Wu, Sun, Li, Jahn and Wilde2002) during the Early Jurassic. However, this magmatic assemblage may also form in a back-arc setting. The intrusion of the DBZG mafic rocks occurred 30–40 Ma later than the onset of final amalgamation between the Siberia and North China Cratons (NCC) during the Late Permian or Early Triassic (Zhou et al. Reference Zhou, Wilde, Zhang and Zhao2009; Wang et al. Reference Wang, Xu, Xu, Gao and Ge2015). We, therefore, propose that emplacement of the Dabaizhgou dolerite pluton was related to subduction of the palaeo-Pacific ocean plate beneath East Asia, rather than the Siberia–NCC collision event.

6.b. New tectonic model

We present a new tectonic model here based on our geochemical and geochronological data and interpretations from the Dabaizhgou pluton, combined with the extant data from other magmatic and metamorphic rock assemblages in the Songnen–Zhuangguagncai mountain range in NE China (Fig. 10). The following observations and boundary conditions are used in the construction of this model:

1. (a) There is a well–developed, NNE-trending magmatic belt with both plutonic and volcanic suites, which show progressive younging in age from the west (201–198 Ma) to the east (186–182 Ma). This eastward-younging, Early Jurassic magmatic system has been widely interpreted as the manifestation of the WNW–directed subduction of the palaeo-Pacific Plate beneath the Asian continent (Wu et al. Reference Wu, Yang, Lo, Wilde, Sun and Jahn2007; Yu et al. Reference Yu, Wang, Xu, Gao and Pei2012; Zhou et al. Reference Zhou, Cao, Wilde, Zhao, Zhang and Wang2014; Qin et al. Reference Qin, Lai, Li, Ju, Zhu and Zhao2016; Xu et al. Reference Xu, Zhang, Shi, Brix, Huhma, Chen, Zhang and Zhou2017; Tang et al. Reference Tang, Xu, Wang and Ge2018; Zheng & Dai, Reference Zheng and Dai2018).

2. (b) The Early Jurassic plutonic rocks display significant changes in their geochemical characteristics and isotopic fingerprints. The relatively oldest (201–198 Ma) monzogranite intrusions in the west have high SiO₂, K₂O and Rb contents, depleted whole-rock Sr–Nd–Pb isotopic compositions, and zircons with depleted Lu–Hf isotopic values, reminiscent of adakite-like melts (Qin et al. Reference Qin, Lai, Li, Ju, Zhu and Zhao2016). Their magmas are considered to have been derived from partial melting of subducted melanges at the subducting plate interface dipping at shallow angles to the WNW (Fig. 10). The younger (186–182 Ma) mafic to felsic plutons in the east have calc-alkaline affinities with LILE enrichment and HFSE depletion patterns, characteristic of island-arc rocks; their magmas were produced by partial melting of a subduction-metasomatized mantle wedge above a steeply dipping palaeo-Pacific slab (Yu et al. Reference Yu, Wang, Xu, Gao and Pei2012). Similarly, the Early Jurassic calc-alkaline volcanic rocks in the Dongning–Wangqing–Hunchun region (Xu et al. Reference Xu, Ge, Pei, Meng, Yu and Yang2008), east of the Dunhua–Mishan fault (eastern Heilongjiang–Jilin Provinces), represent the products of island-arc magmatism that formed above this WNW-dipping subduction zone (Tang et al. Reference Tang, Xu, Wang and Ge2018).

3. (c) From the onset of the subduction of the palaeo-Pacific plate beneath the Asian continent in the latest Triassic – Early Jurassic (~201–198 Ma) and onwards the subducting slab experienced rollback and steepening through time (Deng et al. Reference Deng, Liu, Wang, Dilek and Liang2017; Tang et al. Reference Tang, Xu, Wang and Ge2018). This slab rollback and higher angles of subduction caused lithospheric thinning in the upper plate and produced an extended island-arc – back-arc system (Fig. 10).

4. (d) Rapid slab retreat in the Early Jurassic (from ~200 Ma to 180 Ma) induced mantle return flow and asthenospheric upwelling into the extended upper mantle to fill the slab-free region created by the eastward migration of the slab (Fig. 10; Liu et al. Reference Liu, Levander, Zhai, Porritt and Allen2012). This asthenospheric upwelling led to the partial melting of a fertile lherzolitic source in the garnet–spinel stability field (Fig. 10) and to the production of OIB-like melts, which formed the magmas of the DBZG and other mafic plutons. Heat and melt flux driven by the rising asthenosphere further facilitated the lithospheric extension in the upper continental plate due to thermal weakening.

5. (e) There is an exhumed, latest Triassic – Early Jurassic accretionary prism complex to the east of the 186–182 Ma island-arc rocks that is interpreted to have formed and evolved with the eastward-migrating palaeo-Pacific plate subduction (Wu et al. Reference Wu, Yang, Lo, Wilde, Sun and Jahn2007; Zhou et al. Reference Zhou, Cao, Wilde, Zhao, Zhang and Wang2014). The Heilongjian metamorphic complex in the Jiamusi massif and the Nadanhada terrane to the east (Fig. 1) represent this Early Jurassic accretionary prism complex.

Fig. 10.

An interpretive tectonic model for the development of the Dabaizigou pluton in the Zhangguangcai mountain range in NE China from OIB-type melts originated from partial melting of the asthenospheric mantle. Rollback of the WNW-subducting palaeo-Pacific slab towards the east led to its steepening in time, which in turn triggered mantle corner flow and asthenospheric uprising. Collectively, slab rollback and asthenospheric heat flux caused lithospheric scale extension in the continental upper plate.

Our model depicts an incipient continental back-arc tectonic setting for the magmatic development of the DBZG mafic pluton. However, the DBZG mafic rocks are characterized by OIB-type geochemistry, which is different from typical oceanic back-arc basalts with geochemical features characteristic of both MORB and island-arc volcanic basalts (e.g. Gribble et al. Reference Gribble, Stern, Bloomer, Stüben, O’Hearn and Newman1996; Ma et al. Reference Ma, Cao, Zhou and Zhu2015). Nevertheless, the Quaternary pre- and post-caldera basaltic rocks of the Payún Matrú volcanic field (PMVF) in the back-arc setting of the Southern Andes in western Argentina show geochemical similarities to both the DBZG mafic rocks and OIBs (Hernando et al. Reference Hernando, Aragón, Frei, González and Spakman2014). In tectonic discrimination diagrams (Fig. 8), both the DBZG mafic rocks and the PMVF basalts plot in the within-plate alkaline basalt field, although the PMVF samples display the effects of moderate crustal contamination. Similarly, basalts from the middle Okinawa Trough (west of the Ryukyu Islands), which represents an incipient intracontinental back-arc basin, also display E-MORB-type (or OIB-like) geochemical features (Shinjo et al. Reference Shinjo, Chung, Kato and Kimura1999). Thus, our proposed incipient continental back-arc tectonic setting for the Early Jurassic mafic magmatism in NE China is analogous to some of the well-documented cases of OIB-type volcanism and extensional tectonics in other modern continental back-arc environments.