Petrogenesis of Alkaline Complex of the Longbaoshan Rare Earth Element Deposit in the Luxi Block, North China Craton, China

: The alkaline complex in the southwest region of Luxi Terrane of the North China Craton is spatially correlated with the newly discovered Longbaoshan REE deposit. Its petrogenesis, however, remains ambiguous. In this study, we present an integrated petrology, whole‑rock geochemistry, sphene U‑Pb and rare earth element data from the Longbaoshan alkaline complex to investigate the petrogenesis, magma source and tectonic evolution. The Longbaoshan alkaline complex consists of mafic to intermediate rocks of hornblende diorite and alkaline hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite. The hornblende diorites show a composition of low SiO 2 , high MgO, Fe 2 O 3 and moderate Na 2 O, CaO and are metaluminous and medium‑to‑ high‑K calc‑alkaline. The hornblende syenite porphyries, biotite monzonites and argirine diorite porphyrites display a relatively higher content of SiO 2 , Na 2 O, K 2 O and Al 2 O 3 and lower contents of MgO, Fe 2 O 3 and CaO and are metaluminous, peralkaline, high‑K calcic‑alkaline and shoshonite. The sphene U‑Pb data shows that the parent magma of the hornblende diorite was emplaced at ca. 120 Ma. All these samples show a common depletion in Th, Nb‑Ta and Zr‑Hf and enrichment in large ion lithophile elements (e.g., Pb, Ba, Sr) and Light Rare Earth Elements. The magma may have experienced fractionation of pyroxene, amphibole, sphene, apatite and zircon during its evolution. The variable La content, La/Sm, Rb/Sr and (Ta/Th) N ratios indicate that the parent magma may produce by partial melting of a mantle source that was interacted with sediment‑derived melts in a subduction setting. Therefore, we propose that the parent magma of the Longbaoshan alkaline complex was derived from a lithospheric mantle which was metasomatized by sediment‑derived melt in a prior subduction process. The enriched magma was emplaced through an extension process and experienced subsequent fractionation and assimilation with the continental crust during the rollback of the Paleo Pacific Ocean plate


Introduction
The alkaline rocks occur in a variety of tectonic settings, such as continental rift valleys, divergent continental margins, oceanic and continental intraplates and subduction zones [1][2][3][4]. Generally, the parent magma of the alkaline rocks is derived from an enriched mantle. Therefore, the geochemistry characteristics of these alkaline rocks may preserve the mantle information [5,6]. However, the petrogenesis of the alkaline rocks remains controversial. Previous investigators have suggested that the alkaline magmas may be generated through partial melting of a metasomatized lithospheric mantle without assimilation and fractional crystallization (AFC), or magma mixing or fluid-rock interaction [7][8][9].  [19]. (b) Sketch geological map of the eastern Luxi Terrane, modified after [50,51]. (c) Geological sketch of the Longbaoshan alkaline complex.
The Longbaoshan alkaline complex exposes in an area of ca. 2.5 km 2 in the southeastern part of the Luxi Terrane, west of the south section of Yishu Fault Zone and southeast of Nishan uplift (Figure 1b). From bottom to top, the strata include the Archaean granitic gneisses, Cambrian limestone, Ordovician limestone and dolomite and Quaternary sediments [52]. The late Paleozoic strata are widely distributed throughout the Longbaoshan area. According to the direction distribution, the faults in this region are mainly divided into four groups with an orientation of NW, NNE, EW and NS (Figure 1c).

Sphene LA-ICP-MS U-Pb Dating and REE Composition Analysis
Sphene LA-ICP-MS U-Pb analyses were conducted at the Isotopic Laboratory, Tianjin Center, China Geological Survey. Laser analysis was performed using a Neptune double focusing multiple-collector ICP-MS (Thermo Fisher Ltd.) attached to a NEW WAVE 193 nm-FX ArF Excimer laser-ablation system (ESI Ltd.). The range of mass number is 4-310 amu; resolution greater than 450 (flat peak, 10% peak valley definition); the abundance is less than 5 ppm (without RPQ) and less than 0.5 ppm (with RPQ) [53].
The ion optical path of the multi receiver inductively coupled plasma mass spectrometer used in this dating work adopts the double focusing design of energy focusing and mass focusing and uses dynamic zoom to expand the mass dispersion to 17%. The instrument is equipped with 9 Faraday cup receivers and 4 ion counter receivers. In addition to the central cup and ion counter, the other 8 Faraday cups are configured on both sides of the central cup and accurately adjusted by motor drive. Four ion counters are bound to L4 Faraday cup. The laser ablation system used is the new wave193 nm fxarf excimer laser produced by ESI company of the United States. The wavelength is 193 nm, the pulse width is less than 4 ns and the beam spot diameters are 2, 5, 10, 20, 35, 50, 75, 100 and 150 μm adjustable, pulse frequency 1-200 Hz continuously adjustable, laser output power 15  [19]. (b) Sketch geological map of the eastern Luxi Terrane, modified after [50,51]. (c) Geological sketch of the Longbaoshan alkaline complex.

Sphene LA-ICP-MS U-Pb Dating and REE Composition Analysis
Sphene LA-ICP-MS U-Pb analyses were conducted at the Isotopic Laboratory, Tianjin Center, China Geological Survey. Laser analysis was performed using a Neptune double focusing multiple-collector ICP-MS (Thermo Fisher Ltd.) attached to a NEW WAVE 193 nm-FX ArF Excimer laser-ablation system (ESI Ltd.). The range of mass number is 4-310 amu; resolution greater than 450 (flat peak, 10% peak valley definition); the abundance is less than 5 ppm (without RPQ) and less than 0.5 ppm (with RPQ) [53].
The ion optical path of the multi receiver inductively coupled plasma mass spectrometer used in this dating work adopts the double focusing design of energy focusing and mass focusing and uses dynamic zoom to expand the mass dispersion to 17%. The instrument is equipped with 9 Faraday cup receivers and 4 ion counter receivers. In addition to the central cup and ion counter, the other 8 Faraday cups are configured on both sides of the central cup and accurately adjusted by motor drive. Four ion counters are bound to L4 Faraday cup. The laser ablation system used is the new wave193 nm fxarf excimer laser produced by ESI company of the United States. The wavelength is 193 nm, the pulse width is less than 4 ns and the beam spot diameters are 2, 5, 10, 20, 35, 50, 75, 100 and 150 µm adjustable, pulse frequency 1-200 Hz continuously adjustable, laser output power 15 J/cm 2 . Detailed analytical procedures are given in [54].
In this paper, the sampling method of sample test is point ablation, and the beam spot of laser denudation is 35 µm. The frequency is 8 Hz and the energy density is 11 J/cm 2 . The laser denuded material is sent to ICP-MS with He as the carrier gas. NIST SRM 610 glass and NIST SRM 612 glass were used as external standards to calculate U, Th, and Pb concentrations of sphenes. MKED1 and OLT1 were used as internal standards for monitoring the stability and accuracy of the instrument and acquired U-Pb data. The results of standard measurements are 1519.7 ± 4.4 Ma (n = 12, MSWD = 0.29) for the MKED1 and 1015 ± 4 Ma (n = 12, MSWD = 1.11) for the OLT1, respectively. Every six analyses were followed by two analyses of the standard sphene MKED1 and two analyses of the standard sphene OLT1. Isotopic ratios were calculated using ICPMSDataCal 8.4 [55] and were plotted using Isoplot version 3.0 software [56]. Common Pb corrections were made using the method of [57].

Whole-Rock Major and Trace Elements Analysis
After the removal the altered surfaces, fresh samples were selected, crushed and powdered to less than 200 mesh in an agate mill for whole-rock analysis. Briefly, 1 g of sample was weighed and put into the crucible and baked in a high temperature furnace at 1000 • C for 1h to obtain the loss on ignition (LOI). Major elements were analyzed by X-ray fluorescence using a Axios PW4400 spectrometer at the testing center of Shandong Provincial Lunan Geology and Exploration Institute, China. Trace elements were determined using a Anglient 7900 ICP-MS instruments with analytical uncertainties of 1-3%. Details of the analytical techniques are described by [58]. Analyses of basalt and andesite standards (BHVO-1, BCR-2 and AGV-1) indicated that the analytical precision and accuracy were better than 5% for major elements and 10% for trace elements and REEs.

Petrology and Rock Association
The Longbaoshan alkaline complex is the host rock of the Longbaoshan rare earth element deposit. The alkaline complex includes the hornblende diorite, hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite (Figure 1c) [44]. Among them, the hornblende diorite exposes in the northwest area of Longbaoshan and hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite are exposed in the southeast area of Longbaoshan.  (Figure 3a,b). The plagioclase shows carlsbad and polysynthetic twining, which are analogous to the albite and oligoclase, respectively.

Geochronology and REE Geochemistry of Sphene
The sphene grains from the hornblende diorite (21LBS27) are euhedral to subhedral and range from 70 to 200 µm in size, with length-to-width ratios of 1:1 to 2.5:1. Most of the sphenes show oscillatory zoning in the CL images. Twenty-nine sphene grains were selected for U-Pb analysis. The analyzed spots yield lower intercept age of 120 ± 8.2 Ma on a Tera-Wasserburg diagram (2σ, n = 29, MSWD = 2.4) (Figure 4). They show Th and U contents of 193 to 533 ppm and 28 to 98 ppm, respectively. The Th/U ratios are high and range from 5.0 to 11.2 (Table 1). Minerals 2022, 12, x 6 of 26

Geochronology and REE Geochemistry of Sphene
The sphene grains from the hornblende diorite (21LBS27) are euhedral to subhedral and range from 70 to 200 μm in size, with length-to-width ratios of 1:1 to 2.5:1. Most of the sphenes show oscillatory zoning in the CL images. Twenty-nine sphene grains were selected for U-Pb analysis. The analyzed spots yield lower intercept age of 120 ± 8.2 Ma on a Tera-Wasserburg diagram (2σ, n = 29, MSWD = 2.4) (Figure 4). They show Th and U contents of 193 to 533 ppm and 28 to 98 ppm, respectively. The Th/U ratios are high and range from 5.0 to 11.2 (Table 1).    Total REE contents of the sphene from the hornblende diorite range from 2.4% to 4.4%. The light rare earth element (LREE) content ranges from 2.4% to 4.1%, and that of heavy rare earth (HREE) is 0.16-0.18% ( Figure 5). The chondrite normalized patterns of the sphene are right inclined, with LREE/HREE (without normalization) and (La/Yb) N (normalized to chondrite) ratios ranging from 16 to 42 and 33 to 75, respectively. LREE enrichment and HREE depletion are obvious. Most of the sphenes exhibit weak negative Eu anomalies (n = 26, 0.44-0.89) and a few sphenes exhibit weak positive Eu anomalies (n = 3, 1.07-1.65). All sphenes show positive Ce anomalies (mostly 1.21-1.37) ( Table 2). baoshan alkaline complex.
Total REE contents of the sphene from the hornblende dio 4.4%. The light rare earth element (LREE) content ranges from 2 heavy rare earth (HREE) is 0.16-0.18% ( Figure 5). The chondrite the sphene are right inclined, with LREE/HREE (without norm (normalized to chondrite) ratios ranging from 16 to 42 and 33 to enrichment and HREE depletion are obvious. Most of the sphene Eu anomalies (n = 26, 0.44-0.89) and a few sphenes exhibit weak p = 3, 1.07-1.65). All sphenes show positive Ce anomalies (mostly 1

Whole-Rock Major and Trace Elements Geochemistry
Given the loss on ignition (LOI) values for most of the analyzed samples, major element concentrations were recalculated on an anhydrous basis.
The primitive mantle-normalized trace element patterns reflect distinctly negative anomalies of Th, Ce, Nb-Ta and Zr-Hf and positive anomalies of LILE (Pb, Ba, Sr) and La for these rocks (Figure 7a (Figure 7b). The variable enrichment of REE (1 to 1000 times) indicates that the rare earth elements were partitioned among different minerals during fractional crystallization [59].

Early Cretaceous Alkaline Granite Magmatism
The eastern North China Craton experienced three peak periods of magmatism during the Mesozoic, which were in the Triassic (216-224 Ma), Jurassic (160-176 Ma) and Early Cretaceous (120-136 Ma) [63]. Lan et al. [33] performed zircon LA-ICP-MS U-Pb dating from the quartz syenite, aegirine-augite syenite, hornblende syenite, monzonite and syenodiorite of the Longbaoshan alkaline complex and suggested that the parent magma was emplaced at 130-129 Ma. Zhou et al. [64] conducted zircon SHRIMP U-Pb dating on the quartz syenite of the Longbaoshan alkaline complex and obtained an age of 125 ± 2.2 Ma. The rare earth element patterns of the sphenes from the hornblende diorite of this study show right-inclined characteristics. And most of sphenes show weak negative Eu anomaly (<1.0) and slight positive Ce anomaly (1.2-1.4) with high Th/U ratio of 1.2-2.3 ( Figure 5), which indicates that the sphenes have a magmatic origin. Thus, the sphene U-Pb dating of the 120 ± 8.2 Ma represents the crystallization age of the parent magma of the hornblende diorite. Consequently, the magma emplacement of the Longbaoshan alkaline complex can be constrained at 128-112 Ma, which is consistent with the Early Cretaceous magmatic event in the North China Craton [63].

The Role of Fractional Crystallization and Assimilation
The formation of the alkaline rock is generally influenced by the magma source, fractional crystallization, accumulation with or without crustal assimilation [8,10,[65][66][67][68]. The alkaline complex exposed in the Longbaoshan area shows an absence of vertical zoning, namely the ultramafic rock, gabbro, plagioclase, diorite and granite from bottom to top. In addition, the rocks from the Longbaoshan alkaline complex show a porphyry texture and contain phenocrysts without lineation. Consequently, we suggest that cumulation can be excluded in the formation of these rocks.
The negative correlation between SiO 2 and MgO, Fe 2 O 3 , CaO, Cr and Ni of the samples from the Longbaoshan alkaline complex reflects the removal of pyroxene and amphibole in the early stage of crystallization (Figures 8 and 9). Their low abundance of TiO 2 and the negative correlation of TiO 2 and CaO versus SiO 2 are explained by the crystallization of sphene (Figure 8f). The negative correlation of P 2 O 5 and CaO versus SiO 2 of these rocks indicates the crystallization of apatite (Figure 8g). The Zr abundance shows a decreasing trend with the increasing SiO 2 contents (Figure 9e). This feature is qualitatively explained by the removal of zircon in the late stage of fractionation. The positive Ba and Sr versus SiO 2 of these rocks from the Longbaoshan alkaline complex are interpreted as the fractionation of feldspar [69,70].
Crustal contamination may lead to an increasing trend in K 2 O/TiO 2 and K 2 O/P 2 O 5 ratios [71]. The K 2 O/TiO 2 ratios of the studied samples from Longbaoshan alkaline complex range from 1.3-2.1 (hornblende diorites) to 6.5-12.8 (hornblende syenite porphyries and aegirine diorite porphyrite) and then increase to 15.5-25.3 (biotite monzonite porphyries), and the K 2 O/P 2 O 5 (3.68-79.72) ratios are also variable, which indicates crustal contamination in the formation of the Longbaoshan alkaline complex. The Rb/Nb ratios of the samples from the Longbaoshan alkaline complex vary between 4.3 and 32.2 (mostly 6-7), which are significantly higher than the mantle ratios (0.24-0.89) and close to the crust ratios (5. 36-6.55). This is consistent with a crustal contamination hypothesis. In addition, the inherited zircons (2.51-2.64 Ga) from the Early Cretaceous Longbaoshan alkaline complex show a similar age to those of zircons from Late Archean gneisses of the Luxi Terrane [72], which, together with the negative εHf(t) values of −19.2 to −12.8 indicate that an ancient crustal material may have been involved in the parent magma [33]. The Nb/Th ratios of the alkaline rocks range from 0.5 to 2.0 and are consistent with the crustal derived alkaline rocks (~1.1), which substantiates the involvement of crustal material [31][32][33][34]. tion of sphene (Figure 8f). The negative correlation of P2O5 and CaO versus SiO2 of th rocks indicates the crystallization of apatite (Figure 8g). The Zr abundance shows a creasing trend with the increasing SiO2 contents (Figure 9e). This feature is qualitativ explained by the removal of zircon in the late stage of fractionation. The positive Ba Sr versus SiO2 of these rocks from the Longbaoshan alkaline complex are interpreted the fractionation of feldspar [69,70].

Nature of the Alkaline Magma Source
Although the hornblende diorite, hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite from the Longbaoshan alkaline complex show a diverse composition, their similar ages, whole-rock trace elements patterns suggest that the parent magmas of these rocks were derived from the same magma chamber. the alkaline rocks range from 0.5 to 2.0 and are consistent with the crustal derived alkaline rocks (~1.1), which substantiates the involvement of crustal material [31][32][33][34].

Nature of the Alkaline Magma Source
Although the hornblende diorite, hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite from the Longbaoshan alkaline complex show a diverse composition, their similar ages, whole-rock trace elements patterns suggest that the parent magmas of these rocks were derived from the same magma chamber. Previous studies have shown that the geochemical composition of the lithospheric mantle of the North China Craton has been changed from a LILE-, Pb-and LREE-enriched and Nb-and Ta-depleted mantle to a LILE-and LREE-enriched mantle with no Nb or even an Nb-enriched and Pb-depleted mantle during the Early Cretaceous [75]. The samples from the Longbaoshan alkaline complex are strongly enriched in LREE and LILE, and depleted in Nb, which together with the high Ba/Nb (218.8) and Rb/Nb (9.5) ratios, indicate that the magma was derived from an enriched mantle source. These features are similar to a subduction-related arc-magmatism-produced rocks [76,77]. In the La/Nb versus Ba/Nb plot, the studied samples from the Longbaoshan alkaline complex fall into the arc volcanics field ( Figure. 10a), indicating that the mantle source was generated in a subduction zone. The alkaline rocks of the Longbaoshan alkaline complex show a high (Hf/Sm) N and variable (Ta/Th) N ratios, indicating a subduction-related metasomatic mantle source region ( Figure. 10b). The Rb/Sr ratios of the rocks from the Longbaoshan alkaline complex (average 0.07) are lower than the average continent crust (upper crust: 0.31, lower crust: 0.22) and close to the mantle source (0.03), indicating the parent magma was derived from a mantle source [78]. The Ba/Rb ratios of this complex  are higher than that of the continental crust (8)(9), which is consistent with a crustal assimilation mechanism [79]. The Nb/Ta and La/Nb ratios of the samples from this complex are higher than the continental crust (12-13, La/Nb: 1.7) and are consistent with a mantle source (Nb/Ta: 15.5-19.5, La/Nb > 1.7) [78,79]. In addition, the Longbaoshan alkaline complex shows negative εHf(t) values of −19.2 to −13.5 and εNd(t) values of −15.8 to −11.8, which further suggests the mantle source has been enriched [33].
Since the La/Sm and (La/Yb) N ratios are sensitive to magmatic processes, they can thus be used to determine the role of partial melting and fractional crystallization in the formation of the alkaline complex. The La/Sm and (La/Yb) N ratios increase with the Previous studies have shown that the geochemical composition of the lithospheric mantle of the North China Craton has been changed from a LILE-, Pb-and LREE-enriched and Nb-and Ta-depleted mantle to a LILE-and LREE-enriched mantle with no Nb or even an Nb-enriched and Pb-depleted mantle during the Early Cretaceous [75]. The samples from the Longbaoshan alkaline complex are strongly enriched in LREE and LILE, and depleted in Nb, which together with the high Ba/Nb (218.8) and Rb/Nb (9.5) ratios, indicate that the magma was derived from an enriched mantle source. These features are similar to a subduction-related arc-magmatism-produced rocks [76,77]. In the La/Nb versus Ba/Nb plot, the studied samples from the Longbaoshan alkaline complex fall into the arc volcanics field (Figure 10a), indicating that the mantle source was generated in a subduction zone. The alkaline rocks of the Longbaoshan alkaline complex show a high (Hf/Sm) N and variable (Ta/Th) N ratios, indicating a subduction-related metasomatic mantle source region (Figure 10b). The Rb/Sr ratios of the rocks from the Longbaoshan alkaline complex (average 0.07) are lower than the average continent crust (upper crust: 0.31, lower crust: 0.22) and close to the mantle source (0.03), indicating the parent magma was derived from a mantle source [78]. The Ba/Rb ratios of this complex  are higher than that of the continental crust (8)(9), which is consistent with a crustal assimilation mechanism [79]. The Nb/Ta and La/Nb ratios of the samples from this complex are higher than the continental crust (12-13, La/Nb: 1.7) and are consistent with a mantle source (Nb/Ta: 15.5-19.5, La/Nb > 1.7) [78,79]. In addition, the Longbaoshan alkaline complex shows negative εHf(t) values of −19.2 to −13.5 and εNd(t) values of −15.8 to −11.8, which further suggests the mantle source has been enriched [33].
Since the La/Sm and (La/Yb) N ratios are sensitive to magmatic processes, they can thus be used to determine the role of partial melting and fractional crystallization in the formation of the alkaline complex. The La/Sm and (La/Yb) N ratios increase with the increase of La contents during partial melting, and the La/Sm and (La/Yb) N values remain stable with the increase of La contents during fractional crystallization [80]. The alkaline rocks from the Longbaoshan alkaline complex show a positive correlation between La contents and La/Sm, (La/Yb) N ratios, indicating that partial melting played a significant role in the magma evolution (Figure 11a,b).
increase of La contents during partial melting, and the La/Sm and (La/Yb) N values remain stable with the increase of La contents during fractional crystallization [80]. The alkaline rocks from the Longbaoshan alkaline complex show a positive correlation between La contents and La/Sm, (La/Yb) N ratios, indicating that partial melting played a significant role in the magma evolution (Figures 11a,b). Previous studies suggested that the mantle source can be modified either by dehydration of slab-derived fluids or sediment-derived melts [82,83], which can be distinguished by the incompatible (e.g., Th, Nb, Ta, Ba, Ti and REEs) and compatible elements (e.g., Rb, Sr) [84]. The studied samples show a variable Rb/Y and low Nb/Y ratios, indicating that the mantle source was enriched by sediment-derived melt (Figure 12a). Additionally, the high La/Sm and low Ba/Th ratios (Figure 12b) consistently suggest that an interaction between the mantle source and sediment-derived melts rather than slab-derived fluids. Since high field strength elements (e.g., Nb, Ta) are mainly enriched in the residual rutile and ilmenite phases, the negative anomalies of Nb, Ta and Ti may imply that the occurrence of residual rutile in the mantle source [85,86]. Overall, the magma source of the Longbaoshan alkaline complex was originated from an enriched mantle through partial melting of an enriched mantle leading to the formation of residual rutile, and the mantle source was modified by sediment-derived melt.  Previous studies suggested that the mantle source can be modified either by dehydration of slab-derived fluids or sediment-derived melts [82,83], which can be distinguished by the incompatible (e.g., Th, Nb, Ta, Ba, Ti and REEs) and compatible elements (e.g., Rb, Sr) [84]. The studied samples show a variable Rb/Y and low Nb/Y ratios, indicating that the mantle source was enriched by sediment-derived melt (Figure 12a). Additionally, the high La/Sm and low Ba/Th ratios (Figure 12b) consistently suggest that an interaction between the mantle source and sediment-derived melts rather than slab-derived fluids. Since high field strength elements (e.g., Nb, Ta) are mainly enriched in the residual rutile and ilmenite phases, the negative anomalies of Nb, Ta and Ti may imply that the occurrence of residual rutile in the mantle source [85,86]. Overall, the magma source of the Longbaoshan alkaline complex was originated from an enriched mantle through partial melting of an enriched mantle leading to the formation of residual rutile, and the mantle source was modified by sediment-derived melt.

Tectonic Implications
stable with the increase of La contents during fractional crystallization [80]. The alkaline rocks from the Longbaoshan alkaline complex show a positive correlation between La contents and La/Sm, (La/Yb) N ratios, indicating that partial melting played a significant role in the magma evolution (Figures 11a,b). Previous studies suggested that the mantle source can be modified either by dehydration of slab-derived fluids or sediment-derived melts [82,83], which can be distinguished by the incompatible (e.g., Th, Nb, Ta, Ba, Ti and REEs) and compatible elements (e.g., Rb, Sr) [84]. The studied samples show a variable Rb/Y and low Nb/Y ratios, indicating that the mantle source was enriched by sediment-derived melt (Figure 12a). Additionally, the high La/Sm and low Ba/Th ratios (Figure 12b) consistently suggest that an interaction between the mantle source and sediment-derived melts rather than slab-derived fluids. Since high field strength elements (e.g., Nb, Ta) are mainly enriched in the residual rutile and ilmenite phases, the negative anomalies of Nb, Ta and Ti may imply that the occurrence of residual rutile in the mantle source [85,86]. Overall, the magma source of the Longbaoshan alkaline complex was originated from an enriched mantle through partial melting of an enriched mantle leading to the formation of residual rutile, and the mantle source was modified by sediment-derived melt.

Tectonic Implications
The North China Craton experienced the subduction of the Paleo-Pacific Plate during the Early Jurassic [88][89][90][91]. The younging trend magmatism from the west to the east of the North China Craton suggest a subsequent rollback process of the Paleo-Pacific Plate during the Early Cretaceous [36,92,93], which triggered the upwelling of the asthenosphere, extension and lithospheric thinning as well as associated magmatism [94][95][96][97]. In addition, Zhu et al. [98] suggested that the subducting Pacific plate was retreated ca. 880 km during 130-120 Ma. The magma emplacement age of 128-112 Ma of the Longbaoshan alkaline complex is consistent with the timing of the rollback of the Paleo-Pacific Plate (130-120 Ma) [22,94].
The alkaline rocks of the Longbaoshan alkaline complex are strongly enriched in LREE and LILEs (e.g., Rb, Ba), and depleted in HREE and HFSEs (e.g., Nb, Ta), which are analogous to volcanic arc granites [100,101]. In the tectonic discrimination diagrams, they are plotted in the volcanic arc granites field, indicating that they were formed in an active continental margin setting (Figures 13a-d). Previous studies have shown that the alkaline magma may be generated from continental arc, post-collisional arc, oceanic arc and within-plate settings [102]. The magma from different tectonic settings shows distinct geochemical features, such as the post-collisional arc magma being characterized by higher ratios of Ce/P and lower ratios of Zr/Ce and Ti/Nb than the continental arc magma and the late oceanic arc magma being characterized by higher concentrations of Hf, La and P than the initial oceanic arc magma [102]. The samples from the Longbaoshan alkaline complex fall within the continental and post-collision arc fields (CAP + PAP) (Figure 14a,b), suggesting subduction-collision-and post-collision-related settings. In addition, most of these samples plot in the continental arc and minor fall into the post-collisional arc fields  [99]. (c) Rb versus (Y+Nb) diagram, modified after [99]. (d) Rb versus (Yb+Ta) diagram, modified after [99]. VAG = volcanic arc granites, Syn-COLG = syn-collisional granites, WPG = within-plate granites, ORG = ocean-ridge granites.
The alkaline rocks of the Longbaoshan alkaline complex are strongly enriched in LREE and LILEs (e.g., Rb, Ba), and depleted in HREE and HFSEs (e.g., Nb, Ta), which are analogous to volcanic arc granites [100,101]. In the tectonic discrimination diagrams, they are plotted in the volcanic arc granites field, indicating that they were formed in an active continental margin setting (Figure 13a-d). Previous studies have shown that the alkaline magma may be generated from continental arc, post-collisional arc, oceanic arc and withinplate settings [102]. The magma from different tectonic settings shows distinct geochemical features, such as the post-collisional arc magma being characterized by higher ratios of Ce/P and lower ratios of Zr/Ce and Ti/Nb than the continental arc magma and the late oceanic arc magma being characterized by higher concentrations of Hf, La and P than the initial oceanic arc magma [102]. The samples from the Longbaoshan alkaline complex fall within the continental and post-collision arc fields (CAP + PAP) (Figure 14a,b), suggesting subduction-collision-and post-collision-related settings. In addition, most of these samples plot in the continental arc and minor fall into the post-collisional arc fields (Figure 14c), suggesting that the mantle was enriched in a prior subduction process and the magma was emplaced through a post-collisional extension process. Moreover, in the Nb/Yb versus Th/Yb diagram (Figure 14d), the studied samples fall into the active continental margin field, which further indicates a subduction process [103].
FOR PEER REVIEW 20 of 26 the magma was emplaced through a post-collisional extension process. Moreover, in the Nb/Yb versus Th/Yb diagram (Figure 14d), the studied samples fall into the active continental margin field, which further indicates a subduction process [103]. In summary, the magma source of the Longbaoshan alkaline complex was originated from a lithospheric mantle which was interacted with sediment-derived melts during a prior subduction process. The magma was emplaced through a subsequent extensional process, which was triggered by the rollback of the subducted plate and upwelling of the asthenosphere [30,96,105]. The parent melts of the Longbaoshan alkaline complex were initially generated by partial melting of an enriched lithospheric mantle, which experienced a subsequent fractional crystallization and assimilation with the continental crust rocks.  [104]. (b,c) TiO2-La-Hf diagram and Zr-Nb-Ce/P 2 O 5 diagram, modified after [104,105]. (d) Nb/Yb versus Th/Yb diagram, modified after [96]. CAP = continental arc potassic rocks; PAP = post-collisional arc potassic rocks; IOP = initial oceanic arc potassic rocks; LOP = late oceanic arc potassic rocks.
In summary, the magma source of the Longbaoshan alkaline complex was originated from a lithospheric mantle which was interacted with sediment-derived melts during a prior subduction process. The magma was emplaced through a subsequent extensional process, which was triggered by the rollback of the subducted plate and upwelling of the asthenosphere [30,96,105]. The parent melts of the Longbaoshan alkaline complex were initially generated by partial melting of an enriched lithospheric mantle, which experienced a subsequent fractional crystallization and assimilation with the continental crust rocks ( Figure 15).

Conclusions
(1) The parent magma of the Longbaoshan alkaline complex was crystallized at 128-112 Ma, which is consistent with the Early Cretaceous magmatic event in the North China Craton. (2) The magma of the Longbaoshan alkaline complex was derived from an enriched lithospheric mantle. The lithospheric mantle was metasomatized by sediment-derived melts during a subduction process. (3) The parent magma of the Longbaoshan alkaline complex was emplaced in an extensional setting during the rollback of the subducting plate and experienced a subsequent fractional crystallization and continental crust assimilation process.

Data Availability Statement:
Acknowledgments: We thank Sutita Changsing, Gary Liu, Filip Kostic, the Academic Editor and the three anonymous referees for their constructive and insightful comments, which significantly improved this manuscript. We also thank Academician Prof. Jun Deng, Mr. Deng-Yang He and Mr. Ya-Qi Huang for their comments, which greatly helped to improve the manuscript.

Conflicts of Interest:
The authors declare no conflicts of interest.

Conclusions
(1) The parent magma of the Longbaoshan alkaline complex was crystallized at 128-112 Ma, which is consistent with the Early Cretaceous magmatic event in the North China Craton. (2) The magma of the Longbaoshan alkaline complex was derived from an enriched lithospheric mantle. The lithospheric mantle was metasomatized by sediment-derived melts during a subduction process. (3) The parent magma of the Longbaoshan alkaline complex was emplaced in an extensional setting during the rollback of the subducting plate and experienced a subsequent fractional crystallization and continental crust assimilation process.