Cenozoic potassic volcanic rocks from the Keluo and Wudalianchi volcanic districts, northeast China: origin from the new sub-continental lithospheric mantle (SCLM) metasomatized by potassium-rich fluids from delaminated lower crust

Abstract

A series of Cenozoic potassium-rich volcanic rocks developed in the Xiaoguli-Keluo-Wudalianchi-Erkeshan districts, northeast China. The source region and potassium-rich mechanism of the potassic rocks remain highly disputed. In this paper, the major elements, trace elements, and Sr-Nd-Pb isotopes of the volcanic rocks in Keluo (KL) and Wudalianchi (WDLC) volcanic districts were analyzed systematically. The results show that the volcanic rocks are characterized by high K₂O (4.36wt.%−6.13wt.%), remarkable enrichment in LREEs and LILEs, as well as the strong fractionation of HREEs. The isotopic characteristics with high ⁸⁷Sr/⁸⁶Sr (0.704990–0.705272), low ¹⁴³Nd/¹⁴⁴Nd (0.512306–0.512417), low ²⁰⁶Pb/²⁰⁴Pb (16.546–17.135) and ²⁰⁷Pb/²⁰⁴Pb (15.002–15.783) of the volcanic rocks suggest the involvement of EM-I-type mantle. On the basis of the geochemical characteristics, the potassium-rich volcanic magma originated from the new SCLM forming after delamination of the ancient SCLM, with metasomatism of the potassium-rich fluids released from the ancient lower crust during the Late Mesozoic. The proposed genetic model assumes the source which represented by a phlogopite-bearing garnet peridotite (with modal garnet in the range of 2%–10%) experienced very low degrees (i.e., ~0.5) of partial melting. During Cenozoic, the lithosphere in northeast China was affected by the extension and decompression of continental rift, and the metasomatized SCLM underwent low degree partial melting, resulting in the formation of potassium-rich primitive basaltic magma.

Keywords

northeast China / Keluo-Wudalianchi volcanic districts / Cenozoic potassic volcanic rocks / petrogenesis / sub-continental lithospheric mantle

1 Introduction

Potassium-rich igneous rocks are characterized by high K₂O contents (K₂O > 3wt.%) and K ₂O/Na₂O ratios (molar K₂O/Na₂O > 1), usually accompanied by other geochemical features such as high MgO, Ni, and Cr contents, indicating that they have undergone relatively limited differentiation ( Foley and Peccerillo, 1992). Potassium-rich rocks generally develop in various tectonic settings, including subduction-related environments, such as island arcs (e.g., Indonesia) and post-collisional settings (e.g., Italy and southern Tibet), and intraplate, such as active rifting (e.g., Toro-Ankole Province of the East African Rift) and hotspot regions (e.g., Western Australia, Gaussberg, and Leucite Hills) (Foley and Peccerillo, 1992; Zhang et al., 1995; Murphy et al., 2002; Zou et al., 2003; Davies et al., 2006; Chen et al., 2007; Prelevic and Foley, 2007; Avanzinelli et al., 2007; Chu et al., 2013; Kuritani et al.,2013; Sun et al., 2014; Sun et al., 2015). With the difference of tectonic environment, potassic rocks have distinct petrogenetic mechanisms (Xu et al., 2012; Chu et al., 2013; Kuritani et al., 2013; Tian et al., 2016; Wang et al.,2017; Fan et al., 2021). Sub-continental lithospheric mantle (SCLM) and recycled crustal materials are generally considered as two source endmembers of the potassium-rich volcanic rocks from intraplate region or plate margin. However, the mechanism responsible for recycling of crustal materials into the mantle source is still of considerable debate: lithospheric delamination or plate subduction?

Cenozoic intraplate volcanoes are widely distributed in northeast China, with more than 590 Cenozoic volcanoes (Liu et al., 2001; Meng et al., 2018), making northeast China one of the best places to study the genesis of intraplate volcanoes. Most Cenozoic volcanic rocks in northeast China are sodic volcanic rocks, with the exception of the potassic volcanic rocks in Xiaoguli, Keluo, Wudalianchi, and Erkeshan regions characterized by high K, light rare earth elements (LREEs), and large ion lithophile elements (LILEs). (Fig.1) (Zhang et al., 1995; Zou et al., 2003; Chu et al., 2013; Zhao et al., 2014). Nowadays the sources of these potassic magmas are in dispute, with the main points as follows: 1) the continental lithospheric mantle (Zhang et al., 1995; Chen et al., 2007); 2) the asthenosphere mantle containing delaminated continental lithosphere mantle fragment (Choi et al., 2006); 3) mantle transition zone metasomatized by potassium-rich sediments (Kuritani et al., 2013).

The dominant viewpoint is that the potassium-rich volcanic rocks come directly from the metasomatized lithospheric mantle. However, various sources of metasomatic materials have been proposed, including the ancient subducted sediments in the mantle transition zone (Sun et al., 2015), delamination of the ancient lower crust (Chu et al., 2013), and some uncertain enriched substances in the asthenosphere (Zhang et al., 1995; Zou et al., 2003; Wang et al., 2017).

Recently, Fan et al. (2021) presented a high-resolution, three-dimensional (3-D) crust and upper-mantle S-wave velocity model of northeast China by combining ambient noise and earthquake two-plane wave tomography based on unprecedented regional dense seismic arrays. The model shows that there are imprints of the interaction between asthenospheric low-degree melts and the overlying subcontinental lithospheric mantle (Fan et al., 2021). However, it is not feasible to determine the melt source only by geophysics. In view of this, this paper reports new geochemical data of volcanic rocks in the less studied districts of Keluo and Wudalianchi, such as Nanshan, Dayishan, Huoshaoshan, and Bijiashan. Combining the published geochemical and geophysical data of the potassium-rich volcanic rocks, new insights into the genesis of the potassium-rich volcanic rocks are put forward.

2 Geological background

The Xiaoguli-Keluo-Wudalianchi-Erkeshan districts (northeast China) at the eastern end of the Central Asian orogenic belt (Jahn et al., 2000) are located at the junction of the northern margin of the North China Craton, the south-eastern margin of the Siberian Plate and the subduction zone of the West Pacific Plate (Fig.1(a)) (Ren et al., 2002; Zhang et al., 2006; Zhou et al., 2009). Since the Paleozoic Era, this area has experienced the tectonic evolution of the Paleo-Asian Ocean, the Mongolian-Okhotsk Sea, and circum-Pacific systems (Zhao et al., 1994). During the Paleozoic Era, the Ergun-Xing᾽an, Songneng-Zhangguangcai, and Jiamusi microcontinental blocks converged and joined with the North China Craton and Siberian Craton (Li, 2006; Zhang et al., 2006). During the Mesozoic, the tectonic setting in this area transited from the Mongol-Okhotsk tectonic system to the Pacific tectonic system accompanied by multi-phased tectonic-magmatic processes (Wu et al., 2000; Xu et al., 2013; Meng et al., 2014). During the Cenozoic, northeast China was in an extensional tectonic setting with the formation of continental rift, and with diffuse basaltic magmatism marking a clear compositional change with respect to the previous Mesozoic cycle dominated by acid rocks (Liu et al., 2001). Cenozoic volcanic rocks are widespread in northeast China, covering an area of approximately 50000 km² (Fig.1(a)) (Liu, 1999; Liu et al., 2001).

There is a volcanic belt with a total length of more than 300 km and an area of more than 14000 km² in the narrow zone between the Songliao Basin and the Great Xing᾽an Range in northeast China. It is one of the best preserved Cenozoic volcanic districts in China, including Xiaoguli, Menlu, Keluo, Wudalianchi, and Erkeshan volcanic districts from north to south (Fig.1(b)) (Fan et al., 1999; Li et al., 2012). Over 80% volcanic rocks are distributed in Keluo and Wudalianchi volcanic districts (Zhao et al., 2014), and there are a large amount of mantle xenoliths in the volcanic rocks in that area (Zhang et al., 2000; Zhang et al., 2011). The Keluo volcanic district consists of nearly 40 small volcanic cones, including the Nanshan, Dayishan, Heshan, and Gushan, which cover an area of approximately 350 km² (Fig.1(c)). The younger basaltic pumice and volcanic cinder fell on alluvial gravel beds, while the older basalt was overlaid on the mountain top with a height of 450–600 m as a form of high platform and cap (Liu, 1999).The Keluo volcanic group erupted in two periods. The early eruption occurred during Early Pliocene to Early Pleistocene (4.59–1.51 Ma) and the late eruption occurred during Middle Pleistocene (1.34–0.133 Ma) (Zhao et al., 2014). The Wudalianchi volcanic district located on the northern bank of the Nemuoer River is the product of multi-phased magmatic activities. It consists of 11 shield volcanoes and 15 volcanic cones (Xu, 1997), covering an area of 800 km² (Fig.1(d)). During the early stage, lava flows prevailed, forming a low shield volcano or lava mound. However, during the late stages, the volcanic cone was composed of lava flow and weakly erupted debris (Fan et al., 1999). The volcanic activity of Wudalianchi lasted from Early Pleistocene to Holocene (Liu et al., 2001; Zhao et al., 2014). The Laoheishan and Huoshaoshan volcanoes erupted from 1719 to 1721.

3 Samples and petrography

The samples of the Keluo volcanic group were collected from Dayizishan and Nanshan volcanic cones, which were mainly composed of basanite and melanocratic leucite-basanite. The samples of Wudalianchi were from Bijiashan and Huoshaoshan volcanic cones (Fig.2(a) –Fig.2(c)), consisting of leucite basalt, murambite, and trachybasalt. The samples of volcanic rocks are mainly porphyritic, with phenocrysts accounting for about 4%–10%, and matrix accounting for 90%–96%. The phenocrysts are mainly composed of olivine and clinopyroxene. The olivine is automorphic hexagonal or granular crystal, with the size of about 0.1–0.6 mm. Clinopyroxene occurs as 0.1–0.8 mm automorphic or hypautomorphic crystal. The matrix is mostly cryptocrystalline and a small amount is microcrystalline. Alteration can be seen in some phenocrysts with embay-like erosion edges (Fig.2(d) –Fig.2(f)).

4 Analytical methods

All the samples were analyzed at the Testing Center of Institute of Geology, Ministry of Nuclear Industry in Beijing (TCIGNI). Whole-rock major element contents were determined on fused glass disks by X-ray fluorescence (XRF) using an XRF-PW2404 sequential spectrometer. Sample powders (0.6 g) were fused with Li₂B₄O₇ (6 g) in an automatic bead fusion furnace at 1100°C for 10 min. Loss on ignition was determined by igniting 2 g whole-rock powder at 1100°C for 10 h. The analytical precision was better than 2%–3% relative. Trace element contents (including REE, Rare Earth Elements) were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at TCIGNI using a FINNIGAN MAT I element system. Whole-rock powders (40 mg) were weighed and dissolved in distilled 1 mL HF and 0.5 mL HNO₃ (HNO₃: H₂O = 1:1, in volume ratio) in 7 mL Savillex Teflon screw-cap capsules and then were ultrasonically stirred for 15 min. The solutions were subsequently dried at 150°C and the residue was digested with the same acid solution. Then, the solutions were heated at 170°C for 10 days, dried and redissolved in 2 mL HNO₃ (HNO₃:H₂O = 1:1, in volume ratio). The solutions were heated at 150°C for 5 h and then evaporated, dried, and redissolved in 2 mL HNO₃ at 150°C for 5 h to ensure that the samples were completely dissolved. 1 mL 500 ppb In was added as an internal standard. Finally, the solutions were diluted in 1% HNO₃ to 50 mL. An international standard (GRS1) was subjected to the same procedure in order to monitor the analytical reproducibility. The analytical precision was better than 5% and 10% relative for the elements with contents larger than 10 × 10⁻⁶ and less than 10 × 10⁻⁶, respectively.

For Rb-Sr isotope analyses, sample powders (0.1 g) were spiked with mixed isotope tracers (⁸⁷Rb-⁸⁴Sr for Rb-Sr isotope analyses), then dissolved with a mixed acid (HF + HNO₃ + HClO₄) in Teflon capsules for 24 h. Subsequently, the solutions were evaporated to dryness and the residue was dissolved in 6 mol/L HCl. The solutions were heated to evaporation, dried, and redissolved in 0.5 mol/L HCl. Then, the solutions were centrifugated and the clear liquid was poured into AG50W × 8 (H⁺) cationic ion-exchange resin columns. The Sr fractions were separated using 2.5 mol/L HCl, dried and dissolved in HNO₃ to give solutions for analysis by mass spectrometry. For Sm-Nd isotope analyses, the procedure was as the same as Rb-Sr before eluviation. Sm and Nd were separated from the other REE fractions in solution using 4 mol/L HCL, and Sm and Nd were separated using P₅O₇ extraction and eluviation resin. The collected Nd fractions were then dissolved in HNO₃ to give solutions for analysis by mass spectrometry. The whole procedure blank was less than 0.2 ng for Rb-Sr isotopic analysis and 0.05 ng for Sm-Nd isotopic analysis. Analytical errors for Sr and Nd isotopic ratios are given as 2б. For Pb isotope measurements, Pb was separated from the silicate matrix and purified using AG1 × 8 anionic ion-exchange columns with dilute HBr as eluant. The whole procedure blank was less than 1 ng.

Sr-Nd-Pb isotope analyses were performed on ISOPROBE-T mass spectrometer. The mass fractionation corrections for Sr and Nd isotopic ratios were based on ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively. The international standard NBS987 gave ⁸⁷Sr/⁸⁶Sr = 0.710250 ± 7. The JMC standard yielded ¹⁴³Nd/¹⁴⁴Nd = 0.512109 ± 3. During the period of Pb isotope analysis repeated analyses of the international standard NBS981 yielded ²⁰⁴Pb/²⁰⁶Pb = 0.0591107±2, ²⁰⁷Pb/²⁰⁶Pb = 0.914338±7 and ²⁰⁸Pb/²⁰⁶Pb = 2.164940±15.

5 Results

5.1 Major elements

The major element results of the volcanic rocks from the KL and WDLC volcanic districts are shown in Tab.1. The analyzed samples are rich in alkalis and belong to the group of potassic rocks (Fig.3(a); i.e., K₂O/Na₂O > 1; Foley and Peccerillo, 1992), with K₂O + Na₂O = 7.92wt.% –10.06wt.%, K₂O = 4.36wt.%–6.13wt.%, K₂O/Na₂O = 1.18–1.59, MgO = 4.24wt.%–7.62wt.%, CaO = 5.34wt.%–7.60wt.%, Al₂O₃ = 13.27wt.%–14.36wt.% and SiO₂ = 49.63wt.%–53.23wt.%. In the TAS classification diagram (Fig.3(b)), the KL and WDLC volcanic rocks fall in the fields for basaltic trachyandensite, trachyandensite, phonolitic tephrite, tephriphonolite, and tephrite/basanite, dominated by phonolitic tephrite, in accordance with the data from available literature (Fan and Hooper, 1991; Zhang et al.,1991; Zhang et al., 1995; Zou et al., 2003; Chu et al., 2013). In Harker-type diagrams (Fig.4), the KL and WDLC volcanic rocks (as well those from the neighboring Erkeshan district) show decreasing Fe₂O₃^T and CaO and increasing Al₂O₃, TiO₂, SiO₂ and K₂O with increasing degree of rock evolution (i.e., decreasing MgO contents). The contents of major element oxides in different regions are not exactly the same. For example, the volcanic rocks in WDLC have higher MgO, CaO, and Fe₂O₃^T contents, lower Al₂O₃, TiO₂, K₂O, and SiO₂ contents, while those in KL have higher Al₂O₃, TiO₂, K₂O, and SiO₂ contents, lower MgO, CaO, and Fe₂O₃^T contents, which is consistent with the results of available studies. However, on the whole, the data points in the KL and WDLC districts are relatively coherent in the Harker-type diagrams (Fig.4), indicating the source of magma is unified.

Filled and open symbols represent, respectively, data from this study and the available literature (Fan and Hooper, 1991; Zhang et al., 1991; Zhang et al., 1995; Zou et al., 2003; Chu et al., 2013).

5.2 Trace elements

Trace element contents are presented in Tab.1. The rocks from the KL and WDLC volcanic districts are rich in REEs (such as La, Ce, Pr, and Nd) and LILEs (such as Rb, Ba, and K). The contents of REEs are 340.21‰–433.49‰. The chondrite-normalized diagram of REEs (Fig.5(a)) reveals a right-leaning pattern. The LREEs are rich, with (La/Yb)_N of 34.43–53.80. The fractionation of HREEs (Heavy Rare Earth Elements) is obvious, with average (Sm/Yb)_N ranging from 7.5 to 10.2. There is no Eu negative anomaly. δEu ( = Eu/Eu*) is 0.98–1.00, with an average of 0.98.

The volcanic rocks in WDLC have high Ba (1657×10^–6–2155×10^–6), Sr (1158×10^–6–1618×10^–6) and Ni (58.5×10^–6–169×10^–6) contents, but low Th (6.56×10^–6–8.97×10^–6), U (1.46×10^–6–2.01×10^–6), and Ta (3.09×10^–6–4.29×10^–6) contents. Different samples have slightly different Zr and Hf contents. Nanshan in KL has lower Zr (753×10^–6–867×10^–6) and Hf (15.9×10^–6–18.4×10^–6), while Dayishan has higher Zr (1176.5×10^–6) and Hf (24.85×10^–6). Primitive mantle-normalized diagram (Fig.5(b)) shows that the samples are enriched in LILEs, with evident peaks at Ba, K, Pb, Zr, and Hf. U, Th, Nb, and Ta show slightly negative anomaly. Ce/Pb ranges between 9.94 and 15.96, significantly lower compared with ocean island basalt values (OIB = 25±5, Hofmann, 1986). La/Nb ranges between 1.29 and 1.59 and Ba/Nb between 25.89 and 31.60. Ni, Cr, and MgO have a positive correlation (Fig.4).

5.3 Sr-Nd-Pb isotopes

The Sr-Nd-Pb isotopic compositions for the rocks from the KL and WDLC volcanic districts are listed in Tab.2.

The samples have high ⁸⁷Sr/⁸⁶Sr (0.704990–0.705272), and low ¹⁴³Nd/¹⁴⁴Nd (0.512306–0.512417). ε_Nd is less than 0. ⁸⁷Sr/⁸⁶Sr is negatively correlated with ¹⁴³Nd/¹⁴⁴Nd (Fig.6(a) and Fig.6(b)). Such Sr-Nd composition is remarkably different from that for primitive mantle and oceanic ridge basalt and prone to enriched mantle. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios are about 16.546–17.135, 15.002–15.783, and 36.493–37.228, respectively, suggesting low radioactive lead content. In the ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagram, the rocks from the KL and WDLC volcanic districts fall in the lower left part, just above the northern hemisphere reference line (NHRL) and in the range of the EM I mantle (Fig.6(c)). In the ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagram, the samples also plot above the NHRL and show good linear relationship (Fig.6(d)). Finally, in the ¹⁴³Nd/¹⁴⁴Nd vs. ²⁰⁶Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr vs. ²⁰⁶Pb/²⁰⁴Pb diagrams (Fig.6(e) and Fig.6(f)), the investigated rocks plot close to the EM I mantle, reflecting the characteristics of enriched mantle.

6 Discussion

6.1 Crustal contamination and fractional crystallization

In theory, crustal contamination is likely to be a major process in intraplate volcanic activities as magma derived from the mantle passes through thick crust before eruption. However, the phlogopite-bearing xenoliths in the volcanic rocks from the KL and WDLC volcanic districts (Zhang et al., 2011) indicate that the magma rises too fast to be contaminated by crustal materials. Generally, the addition of crustal material to magma is expected to result in a negative covariance of ⁸⁷Sr/⁸⁶Sr with MgO (Carlson and Hart, 1988). The (⁸⁷Sr/⁸⁶Sr)_i values of the KL and WDLC basalts vary only slightly over the range of MgO, precluding the possibility of significant crustal contamination. The (Nb/Ta)_N ratios of the potassic rocks from KL and WDLC are nearly equal to 1, indicating Nb and Ta are not markedly fractionated, which shows that the potassic volcanic rocks have not suffered significant crustal contamination. Since both Nb and Ta belong to HFSE (High Field Strength Elements) and have similar geochemical behavior, the fractionation of these elements is weak in most magmatic processes (Liang et al., 2009). In addition, the rocks contaminated by crustal materials usually have low Nb/U ratios. The Nb/U of the volcanic rocks from KL and WDLC is 38–42, similar to OIB, much higher than that of crustal rocks. It is commonly believed that Os isotopic composition is quite sensitive to crustal contamination (Xu et al., 2007; Qi and Zhou, 2008; Jung et al., 2011). The characteristics of Os isotope also suggest the volcanic rocks of WDLC experienced slight crustal contamination (with crustal contamination degree of less than 3.5%) (Chu et al., 2013). In fact, the Sr-Nd-Pb-Hf isotopes, lithophilic elements and REEs of basalts are almost not affected by such a low degree of crustal contamination (Chu et al., 2013), so they can still be used to discuss the magma source.

For the potassic rocks in the KL and WDLC districts, the MgO is relatively high, but the Ni (58×10^–6–169×10^–6) and Cr (79×10^–6–334×10^–6) contents vary significantly (Tab.1), indicating the potassic magma has experienced fractional crystallization to some extent. MgO is positively correlated with Cr and Ni (Fig.4) while the CaO/Al₂O₃ ratio decreases with MgO content, which demonstrates that the magma underwent a certain degree of olivine and clinopyroxene fractional crystallization (Fig.7) (Liu et al., 1995). Basalt lacks obvious Eu anomaly or shows weak Eu peak and plagioclase is not visible under the microscope, indicating that magma did not undergo significant plagioclase fractional crystallization. The fractional crystallization may occur at the depth of more than 15 km (Jung and Masberg, 1998).

In summary, the characteristics of the major elements, trace elements, REEs, and Sr-Nd-Pb isotopes of the potassic volcanic rocks from the KL and WDLC volcanic districts show that magmas did not undergo intense crustal contamination or significant crystallization differentiation in shallow crust. Although fractional crystallization may occur in olivine and pyroxene, on the whole, the geochemical characteristics of the samples are still similar to that of the magma source.

6.2 Source characteristic

The volcanic rocks from the KL and WDLC volcanic districts are characterized by high potassium content, low Al₂O₃ content, strong enrichment of LREEs and LILEs and particularly strong fractionation of the HREEs. The average value of (Sm/Yb) _N is 9.03. The low Nd isotope ratio, low radioactive Pb isotope ratio and relatively high Sr isotopic ratio show the characteristics of EM-I-type enriched mantle. However, it is still controversial whether such mantle endmember originates from asthenosphere mantle (Xu et al., 2005; Choi et al., 2006), mantle transition zone (Kuritani et al., 2011, 2013), or continental lithospheric mantle (Fan and Hooper, 1991; Zhang et al., 1995; Zou et al., 2003; Chu et al., 2013; Duan et al., 2020).

The Nb-Ta-Ti negative anomaly in the primitive-mantle-normalized trace elements diagram for potassic volcanic rocks from the KL and WDLC volcanic districts (Fig.5) is inconsistent with the Nb-Ta-Ti positive anomaly represented by the oceanic basalt of normal asthenosphere mantle (OIB and MORB) (Hofmann, 1986). Morever, the Ce/Pb (9.94×10^–6–15.96×10^–6) and Nb/U (37.8×10^–6–42.5×10^–6) ratio of the potassium-rich volcanic rocks are lower than that of oceanic basalt (OIB and MORB) (25 and 47) (Hofmann, 1986), indicating that it is likely a poor explanation of the petrogenesis of potassic rocks by this model.

Geophysical images have confirmed that the modern pacific plate has deeply subducted a long distance to the mantle transition zone (400–600 km) beneath the KL and WDLC districts (Zhao et al., 2011), and the potassic magma source with EM I characteristics may have originated from the mantle transition zone (Dasgupta et al., 2004; Kuritani et al., 2011, 2013). However, the magma temperature in the northern segment of Xiaoguli district near KL and WDLC is lower than that of the mantle transition zone (1250°C), while consistent with the lithosphere mantle (1180°C) (Sun et al., 2014). Consequently, it is impossible that the potassic volcanic rocks were derived from the mantle transition zone.

Crustal contamination has not played an important role in the formation of the KL and WDLC basalts. The data obtained in this study have a series of specific geochemical characteristics, including high K, LREEs, and LILEs contents, low Al₂O₃, relatively high MgO content, particularly strong HREE fractionation (with (Sm/Yb) _N = 7.5−10.2), the characteristics of EM-I-type Sr-Nd-Pb isotope as well as the existence of phlogopite-bearing garnet peridotite xenoliths, which is consistent with previous studies (Zhang et al., 1995; Chu et al., 2013; Wang et al., 2017). As a result, we suggest that the volcanic rocks from the KL and WDLC volcanic districts originated from the lithospheric mantle with phlogopite-bearing garnet peridotite.

However, EM I isotope signatures suggest the existence of a mantle endmember with a time-integrated depletion in U relative to Pb and enrichment in LREEs. The time scale required for the formation of the EM-I-type signature must be longer than 1 Ga for the SCLM (Rehkämper and Hofmann, 1997). Wang et al. (2017) estimated that it requires at least 2.2 Ga to form such a characteristic lithospheric mantle. However, During the Mesozoic and Cenozoic in east China, large-scale delamination and thinning of the lithosphere occurred (Zhu et al., 2012), and the ancient SCLM almost did not exist. Nevertheless, only when the newly formed SCLM is metasomatized can it possibly be the source of potassium-rich volcanic rocks of the KL and WDLC districts. Consequently, we propose that EM-I-type potassic rocks from the KL and WDLC volcanic districts is more likely to be derived from a new SCLM metasomated by early potassium-rich melt or fluids.

6.3 Fluid metasomatism

The existence of a potassium-rich mineral phase in the magma source is a necessary factor for the genesis of the potassic rocks from the KL and WDLC volcanic districts. Phlogopite is characterized by high K, Rb/Sr and Pb/U (Rosenbaum, 1993; Ionov and Hofmann, 1995), which is consistent with the geochemical characteristics of volcanic rocks (high ⁸⁷Sr/⁸⁶Sr and low ²⁰⁶Pb/²⁰⁴Pb). The lherzolite xenoliths in potassic rocks from the KL and WDLC volcanic districts contain phlogopite (Zhang et al., 2011), which confirms that there is residual phlogopite in mantle beneath the study area. Currently, the focus of the debate is whether the metasomatic agents are represented by silicate melts or fluids. Some authors believe that the potassium-rich materials are silicate melts that are generated by the low-degree melting of deep asthenosphere or the delamination of lower crustal materials (Zhang et al., 2000; Zou et al., 2003; Zhang et al., 2011; Chu et al., 2013). Others argue that the potassium-rich materials come from the fluids released from ancient continental subduction sediments (Kuritani et al., 2013; Sun et al., 2015; Wang et al., 2017; Duan et al., 2019). The trace elements in volcanic rocks can be used to identify the metasomatic materials of source (Elburg and Foden, 1999; Guo et al., 2006; Guo et al., 2013; Zhang et al., 2015). When fluid metasomatism occurs, the fluids would carry a large amount of K, Rb, Ba, Sr, and Pb elements with high fluid activity but a small amount of REE, Th, and HFSE (Hf, Nb, Ta, and Zr) with low fluid activity. In contrast, when melt metasomatism occurs, melts would carry a large amount of Th, LILE, and LREE elements. Therefore, the magmatic rocks formed by fluid metasomatism often have high Ba/La, Ba/Th, Pb/U, and Sr/Th ratios, while the magmatic rocks formed by melt metasomatism have high Th/Nd, Th/U, Th/Ba, and Th/Sr ratios. The high Ba/La, Ba/Th, Pb/U, and Sr/Th ratios suggest that mantle sources of the volcanic rocks from the KL and WDLC volcanic districts were affected by fluid metasomatism (Fig.8).

The geochemical characteristics of potassic rocks, such as high K₂O content, abnormally unradiogenic Pb isotopic compositions, and relatively low ⁸⁷Sr/⁸⁶Sr ratios indicate crustal materials such as metasomatic fluids. However, there is no consensus on the origin of potassium-rich fluids. The main ideas include deep asthenospheric mantle (Zhang et al., 2000), subduction zone (Elburg and Foden, 1999), delaminated SCLM (Choi et al., 2006; Zhao et al., 2014), mantle transition zone (Murphy et al., 2002; Kuritani et al., 2013) or delaminated lower continental crust (Chu et al., 2013). Kuritani et al. (2013) considered that the metasomatic fluids were derived from ancient stagnant slabs in the mantle transition zone which were released and returned to the SCLM. Recently, Wang et al. (2017) proposed the potassium-rich fluids from the mantle transition zone entered the asthenosphere, and then metasomatized the SCLM based on the study on magnesium isotopes of the potassic rocks. However, the subduction plate may have lost water when it reached the lower part of the KL and WDLC districts after a long subduction (more than 2000 km), making it impossible to provide metasomatic fluids directly (Zhang et al., 1995; Zou et al., 2003). Even if the subduction plate has carried fluids, it is difficult to release fluids under the temperature-pressure conditions of a mantle transition zone (Mibe et al., 2011). In addition, the enriched characteristics of ²³⁰Th also indicate that the source of potassic rocks has not experienced the fluid metasomatism of oceanic sediments (Zou et al., 2003). Alternatively, the potassium-rich fluids are likely to be released from the delaminated ancient SCLM. The Re depletion ages (2.2−1.9 Ga) of the xenoliths in the Keluo volcano indicate that the metasomatic fluids come from the ancient SCLM (Zhang et al., 2011). Before Paleoproterozoic (> 1.9 Ga), the lithosphere mantle and lower crust in east China were dominated by granulite (Gao et al., 2003; Zheng et al., 2009; Jiang et al., 2013). The granulite is characterized by low Nd, low Pb, and high Sr isotopic ratio, which is similar to EM I mantle (Gao et al., 2004; Liu et al., 2004). The pyroxene and plagioclase minerals in lower crust granulite may contain a certain amount of water (Xia et al., 2006). With the delamination of the Mesozoic lithosphere and lower crust in east China (Gao et al., 2003; Xu et al., 2013; Meng et al., 2014), the granulite in lower crust that delaminates into asthenosphere can release a large amount of fluids. The potassium-rich fluids passed through the asthenosphere and interacted with SCLM, forming the source of potassic rocks.

6.4 Partial melting of source

In general, HREEs (e.g., Yb, Y) are relatively enriched, while LREEs (e.g., La, Ce) are relatively depleted in garnet phase. Magma evolution can be described by the characteristics of REE ratios (e.g., La/Yb, Dy/Yb) (Bogaard and Wörner, 2003). In detail, La/Yb increases with La in partial melting processes, while La/Yb remains stable with the increase of La in a fractional crystallization process. For the volcanic rocks of KL and WDLC, La/Yb increases linearly with the increase of La (Fig.9(a)), indicating that magmatism is mainly controlled by a partial melting process, and the fractional crystallization of olivine and clinopyroxene is only the secondary controlling factor of magma evolution. The (Dy/Er)_N-(La/Sm)_N diagram is used to simulate the equilibrium partial melting process of potassium-rich volcanic rocks with the proportion of garnet instead of depth. It is assumed that the source consists of 15% clinopyroxene, 25% orthopyroxene, 45%−60% olivine, and 1%−15% garnet. The data of primitive mantle and the distribution coefficients are adopted from Sun and McDonough (1989) and Donnelly et al. (2004), respectively (Tab.3). The simulation result indicates that garnet accounts for 2%−10% in the source of phlogopite-bearing garnet peridotite, and that potassic rocks are formed by low degree partial melting (0.5%) of the source (Fig.9(b)).

6.5 Petrogenesis of potassic rocks

In general, the ancient lithospheric mantle of Craton wouldn’t be preserved for a long time because of frequent delamination or replacement by a young lithospheric mantle during geological evolution (Griffin et al., 1999; Aulbach et al., 2004). The ancient lithospheric mantle in the lower part of North China Craton is also the same case (Gao et al., 2002; Wu et al., 2003). In Late Mesozoic, large-scale delamination and thinning of the lithosphere occurred in east China, resulting from the subduction of the Paleo-Pacific Plate. With the asthenosphere upwelled, a new SCLM formed. The fluids were released from lower crust materials that had delaminated into asthenosphere mantle at early stage. When the potassium-rich fluids rose and mixed into the new SCLM, the potassic source formed through the interaction between the new SCLM and the potassium-rich fluids.

In Cenozoic, northeast China was influenced by the subduction of Pacific Plate to East Asian continental margin, and the back-arc basin of the Japan Sea and the Northeast Asia continental rift system of the Songliao-Xialiaohe Graben were gradually formed (Liu, 1989; Ren et al., 2002). The East Asian continental rift system is composed of a group of near-parallel rifts and faults, which are distributed on both flanks of Songliao Graben with NNE-trending. From rift center (Songliao graben) to both flanks (Daxinganling and Changbai Mountains), the volcanic rocks get younger and transit from tholeiites to alkali basalts (Liu et al., 2001). Affected by the subduction and rollback of the Pacific Plate, when the lithosphere thickness beneath the KL and WDLC districts changed dramatically, local turbulent flow occurred in the asthenosphere flow process. This promoted asthenosphere upwelling (Chen et al., 2021) and further led to lithosphere partial melting with potassic rocks formed in the KL and WDLC districts.

7 Conclusions

1) The volcanic rocks from the KL and WDLC volcanic districts are characterized by high potassium, strong enrichment of light rare earth elements (LREEs), and large ion lithophile elements (LILEs), and particularly strong fractionation of the high rare earth elements (HREEs). The high ⁸⁷Sr/⁸⁶Sr (0.704990–0.705272), low ¹⁴³Nd/¹⁴⁴Nd (0.512306–0.512417), and low ²⁰⁶Pb/²⁰⁴Pb (16.546–17.135) and ²⁰⁷Pb/²⁰⁴Pb (15.002–15.783) suggest that the volcanic rocks have the isotopic composition characteristics of EM-I-type mantle.

2) The potassic volcanic rocks from the Keluo and Wudalianchi volcanic districts originated from a new SCLM. The new SCLM underwent the metasomatism of potassium-rich fluids released from the lower crust material that had delaminated into asthenosphere mantle at an early stage. The potassic rocks are formed by low-degree partial melting (0.5%) of the phlogopite-bearing garnet peridotite (with garnet content of about 2%–10%).

3) In Cenozoic, affected by the subduction and rollback of the Pacific plate in northeast China, lithosphere experienced partial melting and potassic rocks formed because of asthenosphere upwelling.

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