Petrogenesis of the Early Cretaceous Hongshan Complex in the Southern Taihang Mountains: Constraints from Element Geochemistry, Zircon U-Pb Geochronology and Hf Isotopes

: The Hongshan complex, located in the southern part of the Taihang Mountains in the central part of the North China Craton, consists of syenite stocks (including fine-grained biotite aegirine syenite, medium-grained aegirine gabbro syenite, coarse-grained aegirine gabbro syenite, syenite pegmatite, and biotite syenite porphyry), with monzo-diorite and monzo-gabbro dikes. This paper presents zircon U-Pb ages and Hf isotope data and whole-rock geochemical data from the Hongshan complex. LA–ICP-MS zircon U–Pb age from the fine-grained biotite aegirine syenite, monzo-diorite, and monzo-gabbro are 129.3 ± 2.0Ma, 124.8 ± 1.3Ma, and 124.1 ± 0.9Ma, respectively, indicating their emplacement in the Early Cretaceous when the North China Craton was extensively reactivated. The monzo-diorite and monzo-gabbro have low SiO 2 contents (48.94–57.75 wt%), total alkali contents (5.2–9.4 wt%), and ε Hf (t) values of − 22.3 to − 18.4 and are enriched in MgO (4.0–8.2 wt%), Al2O3 (14.3–15.8 wt%), light rare earth elements (LREEs) and large ion lithophile elements (LILEs). Interpretation of elemental and isotopic data suggests that the magma of monzo-diorite and monzo-gabbro were derived from partial melting of the enriched lithospheric mantle metasomatized by slab-derived hydrous fluids. Syenites with high alkali (K 2 O + Na 2 O = 9.4–13.0 wt%) and Sr contents (356–1737 ppm) and low Yb contents (0.94–2.65 ppm) are enriched in Al (Al 2 O 3 = 16.4– 19.1 wt%), but depleted in MgO (0.09–2.56 w%), Cr (Avg = 7.16 ppm), Co (Avg = 6.85 ppm) and Ni (Avg = 9.79 ppm), showing the geochemical features of adakitic rocks associated with thickened lower crust. Combining zircon 176 Hf/ 177 Hf ratios of 0.282176 to 0.282359, ε Hf(t) values of − 18.3 to − 11.8 and ε Nd (t) values of − 11.1 to − 8.2, we conclude that the syenite magma was derived from the mixing of the thickened lower crust and the enriched lithospheric mantle magma. These magma processes were controlled by Paleo-Pacific plate subduction and resulted in the destruction and thinning of the central North China Craton. mean typical timing of The is 124.8 ± 1.3 Ma, GD magmatic age 124.1 geochronological emplacement relationship in the field, MD and GD after facies emplacement. these ages emplacement age of syenite diorite, gabbro at two different events: the early syenite magmatic event and the late diorite and gabbro magmatic event. The the two stages approximately 5 Ma, and both events Early


Introduction
The North China Craton (NCC), one of the oldest landmasses on Earth [1], experienced stable development from the formation of the basement at ~1.8 Ga to the Triassic. In the Early Jurassic, the NCC was reactivated and was disturbed as the lithosphere thinned due to the subduction of the Paleo-Pacific plate. The thickness of the NCC decreased from approximately 200 km in the Paleozoic to 80-120 km in the Cenozoic [2][3][4]. The margins and interior of the NCC are associated with Mesozoic large-scale mantlecrust magma activities, accompanied by the uplift and extension of the continental crust [5][6][7]. With the recent implementation of many research programs focusing on the NCC, it has been recognized by many geologists that the NCC was destroyed on a large scale in the Mesozoic [8]. However, there are different views on the dominant mechanism and dynamic background of NCC lithospheric thinning [2,3,[8][9][10][11][12].
The Handan area is located in the southern part of the Taihang Mountains in the central part of the NCC, which is also the westernmost boundary of NCC destruction [3]. The Mesozoic regional magmatic activity is strong [13,14], which is a good place to study the lithospheric thinning mechanism in the central NCC. Traditionally, the alkaline rocks, typical rocks in the lithospheric extensional environment, are the products of the lower crust-mantle interaction during collisional orogenesis and lower crust thinning and are a shallow crust-level manifestation of and a natural "window" to magnify deep geodynamic processes [15][16][17][18][19][20][21]. Therefore, this paper selects the early Cretaceous Hongshan alkaline complex exposed in the southern end of the Taihang Mountains tectonomagmatic belt as the focus of this study [22,23] (Figure 1a). Based on previous studies, we systematically carry out field investigations, the petrology, elemental geochemistry, zircon U-Pb geochronology, and zircon Hf isotopic analysis of the Hongshan complex aimed at constraining the petrogenesis and related crust-mantle process during the craton destruction.

Geological Setting
The NCC is one of the major Archean cratons in eastern Eurasia [24,25] and has experienced geodynamic evolution for at least 3.8 billion years [26]. Based on lithological, geochemical, geochronological, structural, and metamorphic P-T path studies of the basement rocks, it can be divided into Eastern and Western Blocks, separated by Central Orogenic Belt (Figure 1a) [27][28][29][30][31]. The study area is located in the southern part of the Taihang Mountains in the Central Orogenic Belt, sharing boundaries with the crust-level faults of Shanqian and Xibaiyu [32,33]. The regional Archean basement is mainly composed of tonalite-trondhjemite-granodiorite (TTG) gneiss and supracrustal rocks of the Archaean Zanhuang group [34], which is unconformity covered by the young sediments, including the sandstone of Changzhougou Formation in Mesoproterozoic, Cambrian to Ordovician (Paleozoic) marine carbonate rocks, Carboniferous to Permian (Paleozoic) sandstone and Mesozoic volcanic rocks (Figure 1b). The volcanic event of the late Mesozoic formed a large outcrop of intermediate-felsic stocks. It can be divided into three magmatic zones from west to east in this region (including the Shexian complex, the Wu'an complex, and the Hongshan complex) [34] (Figure 1b). The Shexian complex (including Pingshun, Dongye, and Fushan) is distributed in the west and consists of gabbro and diorite. The rocks of the Wu'an complex in the central part (including Qicun, Kuangcun, and Guzhen) are dioritic and monzonitic in composition. The dominant rocks of the Hongshan complex in the east part are syenites [23,35,36]. These rocks were all emplaced in Paleozoic carbonate and sandstone strata.  [36,37]).

Geology and Petrology of the Hongshan Complex
The Hongshan complex is located in the easternmost part of the magmatic belt in the southern Taihang Mountains, with an exposed area of about 50km 2 (Figures 1b and 2). Shen et al. (1977) divided regional intrusive rocks into gabbro diorite-hornblende diorite series, diorite-monzonite series, and alkaline syenite series, which are considered to be the products of homologous magma evolution. After that, many geologists have done much scientific research on the Hongshan complex [23,24,[36][37][38][39]. Recently, based on field geological mapping and borehole logging of Hongshan complex, we have identified syenite, monzo-diorite (MD), and monzo-gabbro (GD) and divided the syenite into fine-grained biotite aegirine syenite (FBAS), medium-grained aegirine gabbro syenite (MAS), coarsegrained aegirine gabbro syenite (CAS), syenite pegmatite (SP) and biotite syenite porphyry (BSP). The geological and petrographic characteristics of each lithofacies are as follows:

Syenite Facies
(1) Fine-grained biotite aegirine syenite (FBAS) This lithofacies occurs as an irregular intrusion with an exposed area of approximately 8 km 2 (Figures 2 and 3a). The rocks are massive and fine-grained (Figure 4a), and the mineral assemblage with sizes of 0.3-0.8mm consists of orthoclase (60-65%), aegirine pyroxene (13-15%), biotite (10-15%), plagioclase (8-12%), with the accessory minerals of magnetite, apatite, and zircon. The majority of orthoclase crystals are either subhedral or granular. The plagioclase is subhedral with the dense polycrystalline twins and belongs to albite. Some aegirine pyroxenes are interstitial between and surrounded by the orthoclase minerals. In addition, the several aegirine pyroxenes are partially replaced by biotite minerals. Nevertheless, the primitive mineral shape of aegirine pyroxenes could be identified.
(2) Medium-grained aegirine gabbro syenite (MAS) This lithofacies is distributed outward to the FBAS, with an approximately 6km 2 outcrop ( Figure 2). The rock-forming minerals with sizes of 1-3mm are orthoclase (75-80%) and aegirine pyroxene (15-20%) with the accessory minerals of the magnetite and zircon (Figures 3b and 4b). The orthoclase is mostly plate-shaped and is overlapped by clay alteration. The aegirine pyroxene is mostly in a euhedral and subhedral columnar shape, and few are fine-grained distributed and are interstitial between and surrounded by the orthoclase minerals.   (3) Coarse-grained aegirine gabbro syenite (CAS) The CAS phase is restricted in the margin of the Hongshan complex and covers an area of approximately 30 km 2 ( Figure 2). The rocks are massive and show a coarse-grained texture (Figures 3c and 4c,d). The minerals (3-6mm in diameters) in these rocks are composed of perthite (orthoclase and albite; 85-90%) and aegirine pyroxene (8-10%), with accessory minerals of apatite, zircon, and magnetite. These perthite minerals show Karnofsky bicrystal and are overlapped by clay minerals. The aegirine pyroxene is a subhedral columnar in shapes and distributes in the interstice between the perthite minerals' interior.
(4) Syenite pegmatite (SP) The lithofacies is enveloped by the FBAS stock and have a vesicle shape (Figures 2  and 3a). The rocks of this phase have massive structure and coarse-grained texture, the rock-forming minerals with diameters of 5-8 mm are orthoclase (65-70%) and perthite (25-30%) (Figure 4e), with the accessory minerals including apatite and zircon.

Samples and Analytical Methods
Fresh and weakly altered syenite, MD, and GD samples were collected from surface and drill core to analyze the whole rock geochemistry, single Zircon U-Pb geochronology, and in situ Hf isotope. The sample location is shown in Figure 2.

Zircon U-Pb Isotope Analyses
LA-ICP-MS completed zircon U-Pb dating in the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources, Jilin University, Changchun, China. See [40] for the specific experimental test process. The laser ablation system is the GeoLasPro 193 nm ArF excimer laser produced by the German COMPEx company. The Agilent 7900 ICP-MS instrument is used in conjunction with the laser. Helium was used as the carrier gas of denudation material in the experiment. The instrument is optimized by using synthetic silicate glass standard reference material NIST610. In situ U-Pb analysis of zircon was carried out by the 91,500 standard zircon external correction method. The laser beam spot for sample analysis was 32 μm in diameter. Isotopic ratios and age values were calculated by GLITTER and Isoplot software. The results were corrected for common lead according to the method of [41].

Major and Trace Elements Analyses of Whole Rock
The whole rock major and trace test of the samples in this paper was completed in the Beijing Research Institute of Uranium Geology, Beijing, China. The main elements were analyzed by X-ray fluorescence with Rigaku Rix 2100 spectrometer, and the analysis uncertainty was 1-5%. The trace elements were analyzed by PEE lan 6000 ICP-MS, and the analysis uncertainty was 1-3%. International reference materials BHVO-1, BCR-2, and AGV-1 are used as test standard samples. The detailed experimental method and test flow are shown in Reference [42].

Zircon Lu-Hf Isotope Analyses
In situ zircon Hf isotope ratio test was completed by LA-MC-ICP-MS in the Wuhan Sample Solution Analytical Technology Co., Ltd, Wuhan, China. The laser ablation system is Geolas HD (Coherent, Santa Clara, CA, USA), and the MC-ICP-MS is Neptune plus (SPSS v8.0, IBM Corp., Armonk, NY, USA). The signal smoothing device is also equipped in the analysis process to improve the isotope ratio test's signal stability and precision [43]. Helium is used as the carrier gas, and a small amount of nitrogen is introduced after the denudation tank to improve the sensitivity of the Hf element [44]. The experiment was carried out in single-point denudation mode. The laser diameter is 44 μm. Refer to [44] for detailed instrument operating conditions and analysis methods.

Zircon U-Pb Dating
The zircon U-Pb data in this study are provided in Table 1, and the representative zircon cathodoluminescence (CL) images and concordia diagrams are shown in

Major Elements, Trace Elements, and REEs
Eighteen samples, including twelve syenite samples (FBAS, MAS, CAS, SP, and BSP), four MD samples, and two GD samples from the Hongshan complex, were analyzed for geochemistry. The results are listed in Table 2

Zircon Lu-Hf Isotopes
The in situ Hf isotope analysis results of zircons from the FBAS, MD, and GD from the Hongshan complex are listed in Table 3 and presented in an εHf(t) vs. age diagram in Figure 10a.

Emplacement Time and Magmatism Stage
The southern part of the Taihang Mountains developed Mesozoic regional alkaline and calc-alkaline magmatic activities with a peak period at 124-154 Ma based on the data of the isotopic geochronology [14,39], and most volcanic events are concentrated at 127-138 Ma [51]. In the Handan area ( Figure 11), we collected 20 age data sets from Pingshun rocks, Dongye rocks, Fushan rocks, Guzhen rocks, Kuangshancun rocks, and Hongshan complex. They are derived from zircon SHRIMP U-Pb ages or zircon LA U-Pb ages of syenite, monzonite, diorite, and gabbro. We can divide them into two peaks of magmatic activity: (1) 135-130 Ma monzonitic-syenite magmatism and (2) ~125 Ma gabbro-diorite magmatism. In this study, the weighted mean ages of zircon grains from the FBAS are 129.3 ± 2.0 Ma (sample ZKS002-8). Zircon grains show typical growth zoning and euhedral grain morphology, suggesting that the ages represent the timing of emplacement of the FBAS. The MD magmatic crystallization age is 124.8 ± 1.3 Ma, and the GD magmatic crystallization age is 124.1 ± 0.9 Ma. The geochronological data are consistent with the emplacement relationship observed in the field, and the MD and GD are the products of another phase of magmatism after syenite facies emplacement. Furthermore, these ages are consistent with the emplacement age of the syenite (135-130 Ma), diorite, and gabbro (~125 Ma) in the Handan area. The above discussion indicates that the Hongshan alkaline complex has experienced at least two different magmatic events: the early syenite magmatic event and the late diorite and gabbro magmatic event. The gap between the two stages of magmatism was approximately 5 Ma, and both events occurred in the Early Cretaceous in the Mesozoic.

Petrogenesis
The geologic and geochronologic characteristics of the Hongshan complex suggest a multistage plutonic and hypabyssal alkaline complex. The syenite, MD, and GD have independent geochemical characteristics. Furthermore, gabbro-diorite magmatism is later than syenite magmatism. These characteristics are difficult to be explained by their origin in the evolution of homologous magma. If they are derived from the same magma evolution, syenite should be from the late magma evolution, and its formation age should be slightly later than gabbro and diorite. This is inconsistent with the dating results. Therefore, we discuss their petrogenesis separately.

Petrogenesis of the Syenites
Petrogenetic models of syenite are diverse, including (1) strong fractional crystallization of mantle-derived basaltic magma [52,53]; (2) partial melting of enriched lithospheric mantle [54,55]; (3) mixing of mantle-derived basaltic magma and crust derived melt [56,57]; and (4) partial melting of thickened continental crust (or partial melting of crustal rocks under high pressure) [58,59]. The syenites from Hongshan are generally high in Al (Al2O3 = 16.38-19.10%), high in Sr (356-1737 ppm), and low in Yb (0.94-2.65 ppm) and have high Sr/Y ratios (17.45-100.55). Furthermore, the syenites have relatively low MgO contents (0.09-2.56%) and are depleted in mantle-derived elements, such as Cr (Avg = 7.16 ppm), Co (Avg = 6.85 ppm), and Ni (Avg= 9.79 ppm). It shows the geochemical properties of adakitic rocks associated with thickened lower crust ( Figure 12) [60]. Experimental pe-trology and phase equilibrium studies show that partial melting of normal-thickness continental crust or the middle-upper part of thickened continental crust (≤1.0 GPa, 30-40 km) produces granitic magma, whereas partial melting of the bottom of thickened continental crust (55-60 km) produces syenite magma [61]. Deng (1998) divided trachyte (syenite) into a high-pressure type and a low-pressure one from the perspective of lithofacies balance. There is no plagioclase in the residual minerals of a high-pressure trachyte melt, which is equivalent to eclogite in terms of mineral assemblage. There is no Eu anomaly in such rocks. Low-pressure trachyte is the product of crystallization differentiation of basaltic magma and has an obvious negative Eu anomaly. The δEu values of the Hongshan syenite vary from 0.72 to 1.23, and there is no obvious negative Eu anomaly. It suggests that there may not be a balance between melt and plagioclase in the magma source area of the Hongshan syenite. The magma of the Hongshan syenite was derived from partial melting under high pressure [62]. The FBAS zircons have high 176 Hf/ 177 Hf ratios (0.282176-0.282359), negative εHf(t) values (−18.3-−11.8) and old two-stage Hf model ages (TDM2) (2.68-3.26 Ga). There are Archean detrital zircons and a few Proterozoic, Paleozoic, and Mesozoic detrital zircons in the FBAS. This shows that the magma may have been derived from the partial melting of the old lower crust of the NCC, and a small amount of upper crustal overburden was incorporated during the process of magma formation and evolution. The difference in Rb/Sr ratios among subtypes of the syenite group is significant. The K-rich CAS has a low value of 0.06-0.07 close to that (0.03) of the primitive mantle [49], whereas Na-rich FBAS has an intermediate value of 0.10-0.11 close to that (0.15) of the continental crust [66]. Both values of CAS and FBAS are lower than the MAS, SP, and BSP (0.13-0.52). The variety in Rb/Sr ratios among these phases is attributed to the introduction of both mantle and crustal materials into the formation of Hongshan syenite magmas, evidenced by Sr-Nd isotopic data of Hongshan syenite. The ( 87 Sr/ 86 Sr)t (t = 135 Ma) values of the Hongshan syenite are 0.70517-0.70735 [37], and the εNd (t) values are −11.1-−8.2. On the ( 87 Sr/ 86 Sr)t-εNd(t) diagram (from [37]), most of the sample points from the Hongshan syenite are located on the evolution line of material exchange between the EMItype mantle and the lower crust.

Petrogenesis of the Diorite and Gabbro
The geological and geochronological characteristics of the diorite and gabbro show that the MD is closely associated with the GD and that they are vein-emplaced syenite. Their mineral compositions and structures are different from that of the other syenites. The rock-forming mineral assemblages of the MD and GD are Pl+Bi+Hb+Or and Pl+Hb, respectively. They are both classified as metaluminous and sodium series rocks. They have the same age of emplacement (124.8 ± 1. " When e is greater than 1, basaltic slab melt forms high silicon adakite after metasomatizing mantle peridotite when passing through mantle wedge; When e is less than 1, the partial melting of mantle wedge peridotite occurs, and the melt formed is modified by slab melt to form low silicon adakite. The MD from Hongshan has low SiO2 (56.10-57.75 wt%) and high TiO2 (0.70-0.77 wt%), which is similar to the characteristics of low silicon adakite [67]. Therefore, we believe these diorites may originate from the partial melting of mantle wedge peridotite metasomatized by subduction slab melt [68,69].
Mafic magmas can be formed in oceanic ridges, ocean islands, mantle plumes, subducted island arcs, continental margins, and intracontinental deep fault environments [70]. The GD from Hongshan has low SiO2 (48.94-48.98 wt%) and high MgO (7.84%-8.16% wt%). Additionally, they show the enrichment characteristics of LILEs and LREEs and have REE patterns similar to that of arc magma [71]. The GD is strongly depleted of high field strength elements such as Nb, Ta, and Ti, with a high Th / Nb ratio (0.289-0.292) and low Nb/La ratio (0.273-0.276), which is very similar to typical arc magma (Th/Nb = 0.11-2.0, Nb/La = 0.15-0.63) [72]. On the diagrams of the Zr versus Ti and Ti versus V( Figure  13a,b), the sample points are located in the range of arc magma on the active continental margin.  [39,75]. This pattern is obviously different from the geochemical characteristics of typical oceanic crust and asthenospheric mantle, which has depleted Nd isotope compositions (εNd(t) value > 0) and low initial 87 Sr/ 86 Sr ratios. However, this pattern is consistent with the characteristics of mafic rocks formed in the enriched lithospheric mantle source area of the Jiaodong area [32]. It is suggested that the magma associated with the diorite and gabbro in the Handan area may have been derived from the enriched lithospheric mantle. Experimental petrological studies show that the hydrous minerals phlogopite and amphibole can only exist stably in the lithospheric mantle [76]. Rb and Ba are incompatible elements in phlogopite, while Rb, Sr, and Ba are moderately compatible in amphibole [77]. This means that melts in equilibrium with amphibole should have significantly lower Rb/Sr ratios (<0.1) and higher Ba/Rb ratios (>20) than melts derived from phlogopite-bearing sources, which have extremely low Ba concentrations and Ba/Rb values (<20) [78]. The MD and GD from Hongshan have low Rb/Sr ratios (0.01-0.06) (excluding alkali metasomatic samples) and high Ba/Rb ratios (24.8-29.9), indicating that they may have been derived from magmas generated by the partial melting of an amphibole-bearing region of the enriched lithospheric mantle (Figure 14b). K/Yb and Dy/Yb can constrain the mantle source's composition and degree of partial melting during the generation of mafic rocks, as well as to discriminate between partial melting in the spinel and garnet stability fields of phlogopite-bearing and/or amphibole-bearing lherzolitic mantle [79,80]. Partial melts generated within the spinel stability field generally have low Dy/Yb values (<1.5), whereas partial melting in the garnet stability field produces melts with high Dy/Yb values (>2.5) (Figure 14a). The MD and GD from Hongshan have Dy/Yb ratios ranging from 1.88 to 2.07, plotting between the partial melting curves of garnet-facies amphibole lherzolite and spinel-facies amphibole lherzolite. This implies that partial melting may have taken place in the spinel-garnet transition zone. The large range of K/Yb values suggests variable degrees of partial melting. Robinson and Wood (1998) demonstrated that the minimum pressure at which garnet is stable on the anhydrous solidus of fertile peridotitic mantle is 2.8 GPa, corresponding to a depth of approximately 85 km, with the maximum depth of the spinel-garnet transition zone being 75-85 km. This indicates that the MD and GD from Hongshan were most likely derived from partial melting of amphibole-bearing lherzolitic lithospheric mantle in the spinel-garnet transition zone at a depth of 75-85 km. There are two main mechanisms for forming enriched lithospheric mantle endmembers by partial melting: metasomatism of melts or fluids from subducted plates and thermal alteration of volatile-rich (CO2 + H2O) from depleted asthenosphere mantle [81,82]. Generally, the fluid-related metasomatism in the subduction process would result in the depletion of Ta and Hf relative to La and Sm, respectively [32]. The depletion of Ta relative to La and enrichment of Hf relative to Sm might be related to the melt-related subduction metasomatism [32]. The low Ta/La ratio and strong depletion in Hf relative to Sm are generally ascribed to the carbonatite metasomatism [83]. The MD and GD from Hongshan have low (Ta/La)N ratios (0.28-0.35) and (Hf/Sm)N ratios (0. 60-1.20). In the diagram of (Ta/La)N-(Hf/Sm)N, the samples fall in the subduction-related fluids metasomatism ( Figure  14c), indicating that the cause of metasomatism may be a fluid-related process. In addition, the diagram of Th/Zr-Nb/Zr also shows the characteristics of enriched mantle modification by subducted slab-derived fluids (Figure 14d).
The above discussion indicates that the MD and GD from Hongshan most likely originated from enriched lithospheric mantle metasomatized by slab-derived hydrous fluids.
The Hongshan syenite may be originated from mixing the thickened lower crust and the enriched lithospheric mantle magma.

Geodynamic Implications
During the Early Cretaceous, mafic magmatism was widely developed in the eastern and central parts of the NCC (including Jiaodong, Liaodong, Luxi, Taihang Mountains, and Dabie Sulu areas) [20,32,[84][85][86]. Accompanied by large-scale magmatic activities, the NCC also developed massive gold mineralization, a large number of metamorphic core complexes, and fault basins [9,87]. These strong magmatic, tectonic, and metallogenic events indicate that the NCC was in an extensional tectonic regime and lost its stability during the early Cretaceous [88].
At present, several models have been proposed to interpret the destruction process of the NCC, including delamination [2] and thermal erosion [89], which are the most widely accepted. The age and genesis of the Hongshan complex may provide some clues for understanding crust-mantle interaction and destruction mechanism, the central part of the NCC. Ma et al. (2016) explained the co-occurrence of the Early Cretaceous asthenospheric and lithospheric mantle-derived mafic rocks in the Jiaodong Peninsula by using the delamination model and considered that the Jiaodong Peninsula and the Bohai Sea were the centers of the lithospheric destruction process of NCC. The rapid and intense lithospheric delamination would trigger thermomechanical erosion within the interior domains of the NCC, such as the Taihang Mountains [86]. The geochemical characteristics of the Hongshan complex indicate that magmas have experienced two processes: partial melting of enriched lithospheric mantle and subsequent mixing with thickened lower crust. In the delamination model, melts of the delaminated lower crust will inevitably interact with the mantle peridotite while migrating upward, resulting in elevated MgO, Cr, and Ni concentrations [2,90]. As mentioned above, the low MgO contents and depleted in Cr, Co, and Ni of Hongshan syenites indicate that it originated from partial melting of the thickened lower crust caused by thermal erosion. It is generally accepted that the destruction of the NCC in the Early Cretaceous was related to the subduction of the Paleo-Pacific plate [2,8,10]. Some studies suggest that the subduction of the Paleo-Pacific plate did not affect the central part of the NCC because of too far away from the coastline, more than 1000 km. However, recent geophysical data show that horizontal subduction of the Paleo-Pacific plate is trapped in the mantle transition zone under the craton [54,91]. The influence of horizontal plate subduction can be as far as 2000 km. Suppose the huge extension of eastern China since Mesozoic and the distance of Japan Sea formed in Cenozoic are removed. In that case, the distance between Taihang Mountain and the ancient Pacific subduction zone should be less than 2000 km [92,93]. The MD and GD in Hongshan are the products of partial melting of lithospheric mantle metasomatized by subducted slab fluid, which corresponds to the subduction of the Paleo-Pacific plate in the central NCC.
Combining the geological, geochronological, geochemical, and isotopic features, we propose a model to interpret the petrogenesis of the Hongshan complex ( Figure 15). Along with the subduction of the Paleo-Pacific plate to the east of Eurasia in the Late Triassic [94], the slab-released fluid modified the overlying mantle. It transformed the Paleozoic cratonic lithospheric mantle to Mesozoic enriched lithospheric mantle. In the Early Cretaceous, the roll-back of the Paleo-Pacific plate led to an extremely extensional environment in the NCC. The enriched lithospheric mantle was heated by upwelling asthenosphere and then partial melting to form the widespread mafic magma chamber in the NCC at a depth of 75-80 km. The magmatic emplacement process resulted in partial melting of the overlying thickened lower crust. Finally, the mixed magma emplaced and formed Hongshan syenite.

Conclusions
The Hongshan rocks are mainly composed of syenite (including the FBAS, MAS, CAS, SP, and BSP), diorite (the MD), and gabbro (the GD). Zircon U-Pb chronology shows that the syenite and the diorite-gabbro emplacements occurred at ~130 Ma and ~125 Ma, respectively. Field geology, petrology, and element geochemistry demonstrate that the parental magmas of Hongshan syenite originated from the mixture of partially melted thickened lower crust and partially melted lithospheric mantle. The Hongshan diorite and gabbro magma most likely originated from enriched lithospheric mantle metasomatized by slab-derived hydrous fluids. They all formed in an extensional environment associated with the subduction and roll-back of the Paleo-Pacific plate beneath the North China plate in the Early Cretaceous.